Display device, display module, electronic device, and manufacturing method of display device

ABSTRACT

One embodiment of the present invention is a display device including a first insulating layer, a second insulating layer, a first transistor, a second transistor, a first light-emitting diode, a second light-emitting diode, and a color conversion layer. The first insulating layer is over the first transistor and the second transistor. The first light-emitting diode and the second light-emitting diode are over the first insulating layer. The color conversion layer is over the second light-emitting diode. The color conversion layer is configured to convert light emitted from the second light-emitting diode into a light having a longer wavelength. The first transistor and the second transistor each include a metal oxide layer and a gate electrode. The metal oxide layer includes a channel formation region. A top surface of the gate electrode is level or substantially level with a top surface of the second insulating layer.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a display device, adisplay module, an electronic device, and a manufacturing methodthereof.

Note that one embodiment of the present invention is not limited to theabove technical field. Examples of the technical field of one embodimentof the present invention include a semiconductor device, a displaydevice, a light-emitting device, a power storage device, a memorydevice, an electronic device, a lighting device, an input device (e.g.,a touch sensor), an input/output device (e.g., a touch panel), a drivingmethod thereof, and a manufacturing method thereof.

2. Description of the Related Art

In recent years, a display device including a micro light-emitting diode(micro LED) as a display element has been proposed (e.g., PatentDocument 1). The display device including micro LEDs as display elementshas advantages of high luminance, high contrast, a long lifetime, andthe like, and has been actively developed as a next-generation displaydevice.

REFERENCE Patent Document

[Patent Document 1] United States Patent Application Publication No.2014/0367705

SUMMARY OF THE INVENTION

The time taken to mount LED chips on a display device including microLEDs as display elements is extremely long, so that manufacturing costreduction is needed.

In a pick-and-place method, for example, red (R), green (G), and blue(B) LEDs are formed on respective wafers, and the LEDs are cut out oneby one and mounted on a circuit board. Therefore, as the number ofpixels of the display device increases, the number of LEDs to be mountedincreases and thus the time taken for mounting becomes longer. Moreover,as the resolution of the display device becomes higher, it becomes moredifficult to mount LEDs.

An object of one embodiment of the present invention is to provide adisplay device with high resolution. Another object of one embodiment ofthe present invention is to provide a display device with high displayquality. Another object of one embodiment of the present invention is toprovide a display device with low power consumption. Another object ofone embodiment of the present invention is to provide a highly reliabledisplay device.

Another object of one embodiment of the present invention is to reducemanufacturing cost of a display device including micro LEDs as displayelements. Another object of one embodiment of the present invention isto manufacture a display device including micro LEDs as display elementswith a high yield.

Note that the description of these objects does not preclude theexistence of other objects. One embodiment of the present invention doesnot need to achieve all the objects. Other objects can be derived fromthe descriptions of the specification, the drawings, and the claims.

One embodiment of the present invention is a display device including afirst insulating layer, a second insulating layer, a first transistor, asecond transistor, a first light-emitting diode, a second light-emittingdiode, and a color conversion layer. The first transistor iselectrically connected to the first light-emitting diode. The secondtransistor is electrically connected to the second light-emitting diode.The first insulating layer is over the first transistor and the secondtransistor. The first light-emitting diode and the second light-emittingdiode are over the first insulating layer. The color conversion layer isover the second light-emitting diode. The color conversion layer isconfigured to convert light emitted from the second light-emitting diodeinto a light having a longer wavelength. The first transistor and thesecond transistor each include a metal oxide layer and a gate electrode.The metal oxide layer includes a channel formation region. A top surfaceof the gate electrode is level or substantially level with a top surfaceof the second insulating layer.

The first transistor preferably further includes a gate insulatinglayer, a first conductive layer, and a second conductive layer. Themetal oxide layer includes a first region overlapping with the firstconductive layer, a second region overlapping with the second conductivelayer, and a third region between the first region and the secondregion. The first conductive layer and the second conductive layer areapart from each other over the metal oxide layer. The second insulatinglayer is over the first conductive layer and the second conductivelayer. The second insulating layer includes an opening overlapping withthe third region. The gate insulating layer is inside the opening andoverlaps with a side surface of the second insulating layer and a topsurface of the third region. The gate electrode is inside the openingand overlaps with the side surface of the second insulating layer andthe top surface of the third region with the gate insulating layertherebetween.

A display device including a first insulating layer, a second insulatinglayer, a first conductive layer, a second conductive layer, a firsttransistor, a second transistor, a first light-emitting diode, a secondlight-emitting diode, and a color conversion layer. The first transistoris electrically connected to the first light-emitting diode through thefirst conductive layer. The second transistor is electrically connectedto the second light-emitting diode through the second conductive layer.The first insulating layer is over the first transistor and the secondtransistor. The first light-emitting diode and the second light-emittingdiode are over the first insulating layer. The first light-emittingdiode includes a first electrode in contact with the first conductivelayer. The second light-emitting diode includes a second electrode incontact with the second conductive layer. A top surface of the firstelectrode and a top surface of the second electrode are level orsubstantially level with a top surface of the second insulating layer.The color conversion layer is over the second light-emitting diode. Thecolor conversion layer is configured to convert light emitted from thesecond light-emitting diode into a light having a longer wavelength. Thefirst transistor and the second transistor each include a metal oxidelayer, and the metal oxide layer includes a channel formation region.

The color conversion layer is preferably in contact with the secondlight-emitting diode. The display device of one embodiment of thepresent invention preferably further including a third insulating layerbetween the second light-emitting diode and the color conversion layer,and the color conversion layer is preferably in contact with the thirdinsulating layer.

The first light-emitting diode and the second light-emitting diode arepreferably each a micro light-emitting diode.

The first light-emitting diode and the second light-emitting diode eachpreferably emit blue light.

The first transistor and the second transistor are preferably differentfrom each other in one or both of a channel length and a channel width.

The display device of one embodiment of the present invention preferablyfurther includes a driver circuit and a fourth insulating layer. It ispreferable that the driver circuit include a circuit transistor, thecircuit transistor include a channel formation region in a semiconductorsubstrate, and the semiconductor substrate overlap with the firsttransistor, the second transistor, the first light-emitting diode, andthe second light-emitting diode with the fourth insulating layertherebetween.

The display device of one embodiment of the present invention preferablyfurther includes a coloring layer over the color conversion layer. Inthis case, the light emitted from the second light-emitting diode ispreferably extracted to the outside of the display device through thecolor conversion layer and the coloring layer.

One embodiment of the present invention is a module including a displaydevice having any of the above structures. The module is provided with aconnector such as a flexible printed circuit (hereinafter referred to asan FPC) or a tape carrier package (TCP) or is mounted with an integratedcircuit (IC) by a chip on glass (COG) method, a chip on film (COF)method, or the like.

One embodiment of the present invention is an electronic deviceincluding the above-described display module and at least one of anantenna, a battery, a housing, a camera, a speaker, a microphone, and anoperation button.

One embodiment of the present invention is a manufacturing method of adisplay device, including steps of forming a plurality of transistors ina matrix over a first substrate, forming a plurality of light-emittingdiodes in a matrix over a second substrate, forming a first conductorelectrically connected to at least one of the plurality of transistorsor at least one of the plurality of light-emitting diodes over the firstsubstrate or the second substrate, bonding the first substrate and thesecond substrate with the first conductor therebetween so that at leastone of the plurality of transistors and at least one of the plurality oflight-emitting diodes are electrically connected to each other, exposinga first surface by separating the second substrate, and forming a colorconversion layer over the first surface. The color conversion layeroverlaps with at least one of the plurality of light-emitting diodes,and planarization treatment is performed at least once in formationprocess of the plurality of transistors. At least one of the pluralityof light-emitting diodes is preferably a micro light-emitting diode. Atleast one of the plurality of transistors preferably includes a metaloxide in a channel formation region.

According to one embodiment of the present invention, a display devicewith high resolution can be provided. According to one embodiment of thepresent invention, a display device with high display quality can beprovided. According to one embodiment of the present invention, adisplay device with low power consumption can be provided. According toone embodiment of the present invention is to provide a highly reliabledisplay device can be provided.

According to one embodiment of the present invention, manufacturing costof a display device including micro LEDs as display elements can bereduced. According to another embodiment of the present invention, adisplay device including micro LEDs as display elements with a highyield can be manufactured.

Note that the description of these effects does not preclude theexistence of other effects. One embodiment of the present invention doesnot need to achieve all the effects. Other effects can be derived fromthe descriptions of the specification, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 shows an example of a display device;

FIGS. 2A to 2C show an example of a method for manufacturing a displaydevice;

FIGS. 3A and 3B show an example of a method for manufacturing a displaydevice;

FIGS. 4A and 4B show an example of a method for manufacturing a displaydevice;

FIGS. 5A and 5B each show an example of a display device;

FIG. 6 shows an example of a display device;

FIGS. 7A and 7B each show an example of a display device;

FIGS. 8A and 8B show an example of a method for manufacturing a displaydevice;

FIGS. 9A and 9B show an example of a method for manufacturing a displaydevice;

FIG. 10A is a top view showing an example of a semiconductor device, andFIGS. 10B to 10D are cross-sectional views showing an example of thesemiconductor device;

FIG. 11A is a top view showing an example of a semiconductor device, andFIGS. 11B to 11D are cross-sectional views showing an example of thesemiconductor device;

FIG. 12 is a circuit diagram showing an example of a pixel of a displaydevice;

FIGS. 13A, 13B, 13C, 13D, and 13E illustrate the classification of thecrystal structures of IGZO, an XRD spectrum of a quartz glass substrate,an XRD spectrum of a crystalline IGZO film, a nanobeam electrondiffraction pattern of the quartz glass substrate, and a nanobeamelectron diffraction pattern of the crystalline IGZO film, respectively;

FIGS. 14A and 14B show an example of an electronic device;

FIGS. 15A and 15B show an example of an electronic device;

FIGS. 16A and 16B show an example of an electronic device;

FIGS. 17A to 17D each show an example of an electronic device; and

FIGS. 18A to 18F show examples of electronic devices.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it will be readily appreciated by those skilled in theart that modes and details of the present invention can be modified invarious ways without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments.

Note that in the structures of the invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description of suchportions is not repeated. The same hatching pattern is used for portionshaving similar functions, and the portions are not denoted by specificreference numerals in some cases.

In addition, the position, size, range, or the like of each structureillustrated in drawings does not represent the actual position, size,range, or the like in some cases for easy understanding. Therefore, thedisclosed invention is not necessarily limited to the position, size,range, or the like disclosed in the drawings.

Note that the terms “film” and “layer” can be used interchangeablydepending on the case or the circumstances. For example, the term“conductive layer” can be changed into the term “conductive film”. Asanother example, the term “insulating film” can be changed into the term“insulating layer”.

Embodiment 1

In this embodiment, a display device of one embodiment of the presentinvention will be described with reference to FIG. 1 , FIGS. 2A to 2C,FIGS. 3A and 3B, FIGS. 4A and 4B, FIGS. 5A and 5B, FIG. 6 , FIGS. 7A and7B, FIGS. 8A and 8B, and FIGS. 9A and 9B.

[Summary of Display Device]

The display device of this embodiment includes a plurality oflight-emitting diodes that are display elements, and a plurality oftransistors for driving the display elements.

In the display device of this embodiment, pixels for different colorsinclude light-emitting diodes that emit light of the same color. In thecase of using blue light-emitting diodes, for example, blue lightemitted from the light-emitting diode in a blue pixel is extracted tothe outside of the display device. Blue light emitted from thelight-emitting diode in a pixel of a color other than blue, such as redor green, is converted by a color conversion layer into light having alonger wavelength than the blue light, and the light is extracted to theoutside of the display device.

In manufacturing a display device in which pixels for different colorsinclude light-emitting diodes that emit light of the same color, onlyone kind of a light-emitting diode is formed over a substrate; thus, amanufacturing apparatus and manufacturing process can be simplifiedcompared to the case where a plurality of kinds of light-emitting diodesare formed. Accordingly, the difficulty in mounting a light-emittingdiode on a circuit board can be lowered. In addition, the difficulty inmounting a transistor and a light-emitting diode on one substrate can belowered.

The display device of one embodiment of the present invention ismanufactured in such a manner that a plurality of transistors and aplurality of light-emitting diodes, which are formed over differentsubstrates, are bonded to each other, the substrate over which theplurality of light-emitting diodes are formed is separated, and a colorconversion layer is provided over a surface exposed by the separation soas to overlap with the light-emitting diodes.

In a manufacturing method of the display device of one embodiment of thepresent invention, a plurality of light-emitting diodes and a pluralityof transistors are bonded to each other at a time, so that themanufacturing time of the display device can be shortened compared tothe case of employing a method in which light-emitting diodes aremounted on a circuit board one by one. Moreover, even in the case ofmanufacturing a display device with a large number of pixels or ahigh-resolution display device, manufacturing difficulty can be lowered.

The display device of this embodiment has a function of displaying animage with the use of a light-emitting diode. In the case where alight-emitting diode, which is a self-luminous element, is used as adisplay element, a backlight is unnecessary and a polarizing plate isnot necessarily provided in the display device. Therefore, the displaydevice can have reduced power consumption and can be thin andlightweight. The display device including a light-emitting diode as adisplay element has high contrast and a wide viewing angle; thus, highdisplay quality can be obtained. Furthermore, with the use of aninorganic material as a light-emitting material, the display device canhave a long lifetime and improved reliability.

In this embodiment, in particular, an example in which a micro LED isused as a light-emitting diode is described. A micro LED having a doubleheterojunction is described in this embodiment. Note that there is noparticular limitation on the light-emitting diode, and for example, amicro LED having a quantum well junction or a nanocolumn LED may beused.

The area of a light-emitting region of the light-emitting diode ispreferably less than or equal to 1 mm², further preferably less than orequal to 10000 μm², still further preferably less than or equal to 3000μm², even further preferably less than or equal to 700 μm². Note that inthis specification and the like, a light-emitting diode including alight-emitting region whose area is less than or equal to 10000 μm² isreferred to as a micro LED in some cases.

The transistor included in the display device preferably includes ametal oxide in a channel formation region. A transistor including ametal oxide consumes less power. Thus, a combination with a micro LEDcan achieve a display device with extremely reduced power consumption.

In particular, the display device of this embodiment preferably includesa transistor in which the top surfaces of a gate electrode and aninsulating layer are level or substantially level with each other. Byplanarization treatment employing a chemical mechanical polishing (CMP)method or the like, for example, the top surfaces of the gate electrodeand the insulating layer are planarized, whereby the top surfaces becomelevel with each other.

A transistor with such a structure can be easily reduced in size. Whenthe size of a transistor is reduced, the size of a pixel can be reduced,so that the resolution of a display device can be increased.

The display device of this embodiment can be manufactured with a highresolution, and thus can be favorably used for an electronic devicehaving a relatively small display portion. Examples of such anelectronic device include a watch-type or bracelet-type informationterminal device (wearable device) and a wearable device capable of beingworn on a head, such as a virtual reality (VR) device such as a headmounted display, a glasses-type augmented reality (AR) device, and amixed reality (MR) device.

[Structure Example 1 of Display Device]

FIG. 1 is a cross-sectional view of a display device 100A. FIGS. 2A to2C, FIGS. 3A and 3B, and FIGS. 4A and 4B are cross-sectional viewsillustrating a method for manufacturing the display device 100A.

In this embodiment, an example is described in which a light-emittingdiode 110 a and a light-emitting diode 110 b are provided in a red pixeland a blue pixel, respectively.

The light-emitting diodes 110 a and 110 b have the same structure andemit light of the same color. In this embodiment, the case where thelight-emitting diodes 110 a and 110 b emit blue light will be describedas an example.

As illustrated in FIG. 1 , light emitted from the light-emitting diode110 a in the red pixel is converted from blue light into red light by acolor conversion layer CCR, the purity of the red light is improved by acoloring layer CFR, and the red light is emitted to the outside of thedisplay device 100A.

Similarly, although not illustrated, light emitted from a light-emittingdiode in a green pixel is converted from blue light into green light bya color conversion layer, the purity of the green light is improved by agreen coloring layer, and the green light is emitted to the outside ofthe display device 100A.

In contrast, light emitted from the light-emitting diode 110 b in theblue pixel is emitted to the outside of the display device 100A withoutpassing through a color conversion layer.

The display device 100A illustrated in FIG. 1 is manufactured in such amanner that an LED substrate 150A illustrated in FIG. 2A and a circuitboard 150B illustrated in FIG. 2B are bonded to each other (FIG. 2C andFIG. 3A), a substrate 101 over which the light-emitting diodes 110 a and110 b are formed is separated (FIG. 3B and FIG. 4A), and the colorconversion layer CCR, the coloring layer CFR, and the like are providedover a surface exposed by the separation (FIG. 4B).

Hereinafter, a structure and a manufacturing method of the displaydevice 100A will be described with reference to FIGS. 2A to 2C, FIGS. 3Aand 3B, and FIGS. 4A and 4B.

FIG. 2A is a cross-sectional view of the LED substrate 150A.

The LED substrate 150A includes the substrate 101, the light-emittingdiode 110 a, the light-emitting diode 110 b, and a protective layer 102.

The light-emitting diode 110 a and the light-emitting diode 110 b eachinclude an electrode 112, a semiconductor layer 113, a light-emittinglayer 114, a semiconductor layer 115, and an electrode 116.

The electrode 112 is electrically connected to the semiconductor layer113. The electrode 116 is electrically connected to the semiconductorlayer 115. The protective layer 102 is provided to cover the substrate101, the semiconductor layer 113, the light-emitting layer 114, and thesemiconductor layer 115. The protective layer 102 covers the sidesurfaces of the electrodes 112 and 116 and have openings overlappingwith the top surfaces of the electrodes 112 and 116. In the openings,the top surfaces of the electrodes 112 and 116 are exposed.

The light-emitting layer 114 is positioned between the semiconductorlayer 113 and the semiconductor layer 115. In the light-emitting layer114, electrons and holes are combined to emit light. One of thesemiconductor layers 113 and 115 is an n-type semiconductor layer, andthe other is a p-type semiconductor layer.

A stacked-layer structure including the semiconductor layer 113, thelight-emitting layer 114, and the semiconductor layer 115 is formed soas to emit, for example, red, yellow, green, or blue light. A compoundof gallium and phosphorus, a compound of gallium and arsenic, a compoundof gallium, aluminum, and arsenic, a compound of aluminum, gallium,indium, and phosphorus, gallium nitride, a compound of indium andgallium nitride, a compound of selenium and zinc, or the like can beused for the stacked-layer structure. In this embodiment, thestacked-layer structure is formed so as to emit blue light.

As the substrate 101, for example, a single crystal substrate ofsapphire (Al₂O₃), silicon carbide (SiC), silicon (Si), or galliumnitride (GaN) can be used.

FIG. 2B is a cross-sectional view of the circuit board 150B.

The circuit board 150B includes a substrate 151, an insulating layer152, a transistor 120 a, a transistor 120 b, a conductive layer 184 a, aconductive layer 184 b, a conductive layer 187, a conductive layer 189,an insulating layer 186, a conductive layer 190 a, a conductive layer190 b, a conductive layer 190 c, and a conductive layer 190 d. Thecircuit board 150B further includes insulating layers such as insulatinglayers 162, 181, 182, 183, and 185. One or more of the insulating layersare sometimes considered as components of a transistor, but are notincluded as components of a transistor in the description in thisembodiment.

An insulating substrate such as a glass substrate, a quartz substrate, asapphire substrate, or a ceramic substrate; a single crystalsemiconductor substrate or a polycrystalline semiconductor substrateincluding silicon, silicon carbide, or the like as a material; acompound semiconductor substrate of silicon germanium or the like; or asemiconductor substrate such as an silicon on insulator (SOI) substratecan be used as the substrate 151.

The substrate 151 preferably blocks visible light (has anon-transmitting property with respect to visible light). When thesubstrate 151 blocks visible light, entry of light from the outside intothe transistors 120 a and 120 b formed over the substrate 151 can beinhibited. However, one embodiment of the present invention is notlimited thereto, and the substrate 151 may have a transmitting propertywith respect to visible light.

The insulating layer 152 is provided over the substrate 151. Theinsulating layer 152 functions as a barrier layer that preventsdiffusion of impurities such as water and hydrogen from the substrate151 into the transistors 120 a and 120 b and release of oxygen from ametal oxide layer 165 to the insulating layer 152 side. As theinsulating layer 152, for example, a film in which hydrogen and oxygenare less likely to be diffused than in a silicon oxide film can be used.Examples of such a film include an aluminum oxide film, a hafnium oxidefilm, and a silicon nitride film.

The transistors 120 a and 120 b each include a conductive layer 161, aninsulating layer 163, an insulating layer 164, the metal oxide layer165, a pair of conductive layers 166, an insulating layer 167, aconductive layer 168, and the like.

The metal oxide layer 165 includes a channel formation region. The metaloxide layer 165 includes a first region overlapping with one of the pairof conductive layers 166, a second region overlapping with the other ofthe pair of conductive layers 166, and a third region between the firstregion and the second region.

The conductive layer 161 and the insulating layer 162 are provided overthe insulating layer 152, and the insulating layers 163 and 164 areprovided to cover the conductive layer 161 and the insulating layer 162.The metal oxide layer 165 is provided over the insulating layer 164. Theconductive layer 161 functions as a gate electrode, and the insulatinglayers 163 and 164 each function as a gate insulating layer. Theconductive layer 161 overlaps with the metal oxide layer 165 with theinsulating layers 163 and 164 therebetween. The insulating layer 163preferably functions as a barrier layer like the insulating layer 152.As the insulating layer 164 in contact with the metal oxide layer 165,an oxide insulating film such as a silicon oxide film is preferablyused.

Here, the top surface of the conductive layer 161 is level orsubstantially level with the top surface of the insulating layer 162.For example, an opening is provided in the insulating layer 162, theconductive layer 161 is formed to fill the opening, and planarizationtreatment is performed by a CMP method or the like, whereby the topsurfaces of the conductive layer 161 and the insulating layer 162 canbecome level with each other. Accordingly, the size of the transistors120 a and 120 b can be reduced.

The pair of conductive layers 166 is provided over the metal oxide layer165 so that the conductive layers 166 are apart from each other. Thepair of conductive layers 166 functions as a source and a drain. Theinsulating layer 181 is provided to cover the metal oxide layer 165 andthe pair of conductive layers 166, and the insulating layer 182 isprovided over the insulating layer 181. An opening reaching the metaloxide layer 165 is provided in the insulating layers 181 and 182, andthe insulating layer 167 and the conductive layer 168 are provided tofill the opening. The opening overlaps with the third region. Theinsulating layer 167 overlaps with the side surfaces of the insulatinglayers 181 and 182. The conductive layer 168 overlaps with the sidesurfaces of the insulating layers 181 and 182 with the insulating layer167 therebetween. The conductive layer 168 functions as a gateelectrode, and the insulating layer 167 functions as a gate insulatinglayer. The conductive layer 168 overlaps with the metal oxide layer 165with the insulating layer 167 therebetween.

Here, the top surface of the conductive layer 168 is level orsubstantially level with the top surface of the insulating layer 182.For example, the top surfaces of the conductive layer 168 and theinsulating layer 182 can be level with each other by providing anopening in the insulating layer 182, forming the insulating layer 167and the conductive layer 168 to embed the opening, and performingplanarization treatment. Accordingly, the size of the transistors 120 aand 120 b can be reduced.

The insulating layers 183 and 185 are provided to cover the top surfacesof the insulating layers 182 and 167 and the conductive layer 168. Theinsulating layers 181 and 183 each preferably function as a barrierlayer like the insulating layer 152. When the pair of conductive layers166 is covered with the insulating layer 181, oxidation of the pair ofconductive layers 166 due to oxygen contained in the insulating layer182 can be inhibited.

A plug electrically connected to one of the pair of conductive layers166 and the conductive layer 187 is provided to fill an opening in theinsulating layers 181, 182, 183, and 185. The plug preferably includesthe conductive layer 184 b in contact with the side surface of theopening and the top surface of one of the pair of conductive layers 166,and the conductive layer 184 a embedded inside the conductive layer 184b. Here, a conductive material in which hydrogen and oxygen are lesslikely to be diffused is preferably used for the conductive layer 184 b.

The conductive layer 187 is provided over the insulating layer 185, andthe insulating layer 186 is provided over the conductive layer 187. Anopening reaching the conductive layer 187 is provided in the insulatinglayer 186, and the conductive layer 189 is provided to fill the opening.The conductive layer 189 functions as a plug for electrically connectingthe conductive layer 187 and the conductive layer 190 a or 190 c.

One of the pair of conductive layers 166 of the transistor 120 a iselectrically connected to the conductive layer 190 a through theconductive layers 184 a, 184 b, 187, and 189.

Similarly, one of the pair of conductive layers 166 of the transistor120 b is electrically connected to the conductive layer 190 c throughthe conductive layers 184 a, 184 b, 187, and 189.

Examples of a material that can be used for the conductive layersincluded in the display device in this embodiment include metals such asaluminum, titanium, chromium, nickel, copper, yttrium, zirconium,molybdenum, silver, tantalum, and tungsten, or an alloy containing anyof these metals as its main component. A single layer structure orstacked-layer structure including a film containing any of thesematerials can be used. For example, the following structures can begiven: a single-layer structure of an aluminum film containing silicon,a two-layer structure in which an aluminum film is stacked over atitanium film, a two-layer structure in which an aluminum film isstacked over a tungsten film, a two-layer structure in which a copperfilm is stacked over a copper-magnesium-aluminum alloy film, a two-layerstructure in which a copper film is stacked over a titanium film, atwo-layer structure in which a copper film is stacked over a tungstenfilm, a three-layer structure in which a titanium film or a titaniumnitride film, an aluminum film or a copper film, and a titanium film ora titanium nitride film are stacked in this order, and a three-layerstructure in which a molybdenum film or a molybdenum nitride film, analuminum film or a copper film, and a molybdenum film or a molybdenumnitride film are stacked in this order. Note that an oxide such asindium oxide, tin oxide, or zinc oxide may be used. Copper containingmanganese is preferably used because controllability of the shape byetching is increased.

Examples of a material that can be used for the insulating layersincluded in the display device of this embodiment include resin such asan acrylic resin, a polyimide resin, an epoxy resin, or a siliconeresin, and an inorganic insulating material such as silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, or aluminumoxide.

In this specification and the like, silicon oxynitride contains moreoxygen than nitrogen. Silicon nitride oxide contains more nitrogen thanoxygen.

Note that the circuit board 150B may include one or both of a reflectivelayer that reflects light of a light-emitting diode and a light-blockinglayer that blocks the light.

As illustrated in FIG. 2C and FIG. 3A, the electrodes 116 and 112 of thelight-emitting diode 110 a provided in the LED substrate 150A areelectrically connected to the conductive layers 190 a and 190 b providedin the circuit board 150B, respectively. In a similar manner, theelectrodes 116 and 112 of the light-emitting diode 110 b provided in theLED substrate 150A are electrically connected to the conductive layers190 c and 190 d provided in the circuit board 150B, respectively.

For example, the electrode 116 of the light-emitting diode 110 a and theconductive layer 190 a are electrically connected to each other througha conductor 117 a. Thus, the transistor 120 a and the light-emittingdiode 110 a can be electrically connected to each other. Similarly, theelectrode 116 of the light-emitting diode 110 b and the conductive layer190 c are electrically connected to each other through a conductor 117c. Thus, the transistor 120 b and the light-emitting diode 110 b can beelectrically connected to each other. The electrode 116 functions as apixel electrode of the light-emitting diode 110 a or 110 b.

The electrode 112 of the light-emitting diode 110 a and the conductivelayer 190 b are electrically connected to each other through a conductor117 b. The electrode 112 of the light-emitting diode 110 b and theconductive layer 190 d are electrically connected to each other througha conductor 117 d. The electrode 112 functions as a common electrode ofthe light-emitting diode 110 a or 110 b.

For the conductors 117 a to 117 d, for example, a conductive paste ofsilver, carbon, copper, or the like or a bump of gold, solder, or thelike can be favorably used. Furthermore, for the electrodes 112 and 116and the conductive layers 190 a to 190 d each of which is connected toany of the conductors 117 a to 117 d, a conductive material having a lowcontact resistance with the conductors 117 a to 117 d is preferablyused. When a silver paste is used for the conductors 117 a to 117 d, forexample, a conductive material connected to the conductors is preferablyaluminum, titanium, copper, an alloy containing silver (Ag), palladium(Pd), and copper (Cu) (Ag—Pd—Cu (APC)), or the like, which has a lowcontact resistance with the conductors.

FIG. 2C shows an example in which the conductors 117 a to 117 d areprovided on the circuit board 150B side, and the LED substrate 150A andthe circuit board 150B are bonded to each other. Alternatively, theconductors 117 a to 117 d are provided on the LED substrate 150A sideand the LED substrate 150A and the circuit board 150B are bonded to eachother.

Note that a plurality of light-emitting diodes may be electricallyconnected to one transistor.

A space between the LED substrate 150A and the circuit board 150B ispreferably filled with a filler layer 125. With the filler layer 125,the adhesion (bonding strength) between each light-emitting diode andthe circuit board 150B can be increased. For the filler layer 125, avariety of kinds of resins such as an acrylic resin, a polyimide resin,an epoxy resin, or a silicone resin can be used.

A phenomenon in which light emitted from the light-emitting diodereaches an adjacent pixel through the filler layer 125 and is emitted tothe outside of the display device may occur (this phenomenon is alsoreferred to as crosstalk). In view of the above, a colored resin such asa black resin or a brown resin is preferably used for the filler layer125. A resin containing carbon black can be used for the filler layer125, for example. This can inhibit light leakage to an adjacent pixel,so that the display quality of the display device can be improved.

In the case where the substrate 101 is separated with the use of laserlight in a later step, the filler layer 125 preferably absorbs the laserlight. As a result, a variety of circuits and elements formed in thecircuit board 150B can be inhibited from being damaged by laser light.

After bonding the LED substrate 150A and the circuit board 150B, thesubstrate 101 is preferably separated. There is no particular limitationon the separation method of the substrate 101.

FIG. 3B shows an example in which the entire surface of the substrate101 is irradiated with laser light 128. In the case where a sapphiresubstrate is used as the substrate 101 and gallium nitride is used forthe semiconductor layer 113, for example, the substrate 101 can beseparated by being irradiated with ultraviolet light as the laser light128 (FIG. 4A).

As the laser, an excimer laser, a solid laser, or the like can be used.For example, a diode-pumped solid-state laser (DPSS) may be used.

A separation layer may be provided between the substrate 101 and thelight-emitting diodes 110 a and 110 b. After the substrate 101 isseparated, the light-emitting diodes 110 a and 110 b may be exposed, orthe separation layer may be exposed. The separation layer may be one ofcomponents of the display device 100A.

The separation layer can be formed using an organic material or aninorganic material.

Examples of an organic material that can be used for the separationlayer include a polyimide resin, an acrylic resin, an epoxy resin, apolyamide resin, a polyimide-amide resin, a siloxane resin, abenzocyclobutene-based resin, and a phenol resin.

Examples of an inorganic material that can be used for the separationlayer include a metal containing an element selected from tungsten,molybdenum, titanium, tantalum, niobium, nickel, cobalt, zirconium,zinc, ruthenium, rhodium, palladium, osmium, iridium, and silicon, analloy containing the element, and a compound containing the element. Acrystal structure of a layer containing silicon may be amorphous,microcrystal, or polycrystal.

Next, as illustrated in FIG. 4B, the color conversion layer CCR isformed over the light-emitting diode 110 a. The red coloring layer CFRis preferably formed over the color conversion layer CCR.

The color conversion layer CCR has a function of converting blue lightinto red light. The color conversion layer CCR can be provided incontact with the light-emitting diode 110 a. Alternatively, aninsulating layer may be provided between the light-emitting diode 110 aand the color conversion layer CCR.

A blue coloring layer may be formed over the light-emitting diode 110 b.When a blue coloring layer is provided, the purity of blue light can beimproved. In the case where a blue coloring layer is not provided, themanufacturing process can be simplified.

As the color conversion layer, a phosphor or a quantum dot (QD) ispreferably used. In particular, a quantum dot has an emission spectrumwith a narrow peak, so that emission with high color purity can beobtained. Accordingly, the display quality of the display device can beimproved.

The color conversion layer can be formed by a droplet discharge method(e.g., an inkjet method), a coating method, an imprinting method, avariety of printing methods (screen printing or offset printing), or thelike. A color conversion film such as a quantum dot film may also beused.

For processing a film to be the color conversion layer, aphotolithography method is preferably employed. Examples of aphotolithography method include a method in which a resist mask isformed over a thin film to be processed, the thin film is processed byetching or the like, and the resist mask is removed, and a method inwhich a photosensitive thin film is formed, and the photosensitive thinfilm is exposed to light and developed to be processed into a desiredshape. For example, a thin film is formed using a material in which aquantum dot is mixed with a photoresist, and the thin film is processedby a photolithography method, whereby an island-shaped color conversionlayer can be formed.

There is no limitation on a material of a quantum dot, and examplesinclude a Group 14 element, a Group 15 element, a Group 16 element, acompound of a plurality of Group 14 elements, a compound of an elementbelonging to any of Groups 4 to 14 and a Group 16 element, a compound ofa Group 2 element and a Group 16 element, a compound of a Group 13element and a Group 15 element, a compound of a Group 13 element and aGroup 17 element, a compound of a Group 14 element and a Group 15element, a compound of a Group 11 element and a Group 17 element, ironoxides, titanium oxides, spinel chalcogenides, and semiconductorclusters.

Specific examples include, but are not limited to, cadmium selenide;cadmium sulfide; cadmium telluride; zinc selenide; zinc oxide; zincsulfide; zinc telluride; mercury sulfide; mercury selenide; mercurytelluride; indium arsenide; indium phosphide; gallium arsenide; galliumphosphide; indium nitride; gallium nitride; indium antimonide; galliumantimonide; aluminum phosphide; aluminum arsenide; aluminum antimonide;lead selenide; lead telluride; lead sulfide; indium selenide; indiumtelluride; indium sulfide; gallium selenide; arsenic sulfide; arsenicselenide; arsenic telluride; antimony sulfide; antimony selenide;antimony telluride; bismuth sulfide; bismuth selenide; bismuthtelluride; silicon; silicon carbide; germanium; tin; selenium;tellurium; boron; carbon; phosphorus; boron nitride; boron phosphide;boron arsenide; aluminum nitride; aluminum sulfide; barium sulfide;barium selenide; barium telluride; calcium sulfide; calcium selenide;calcium telluride; beryllium sulfide; beryllium selenide; berylliumtelluride; magnesium sulfide; magnesium selenide; germanium sulfide;germanium selenide; germanium telluride; tin sulfide; tin selenide; tintelluride; lead oxide; copper fluoride; copper chloride; copper bromide;copper iodide; copper oxide; copper selenide; nickel oxide; cobaltoxide; cobalt sulfide; iron oxide; iron sulfide; manganese oxide;molybdenum sulfide; vanadium oxide; tungsten oxide; tantalum oxide;titanium oxide; zirconium oxide; silicon nitride; germanium nitride;aluminum oxide; barium titanate; a compound of selenium, zinc, andcadmium; a compound of indium, arsenic, and phosphorus; a compound ofcadmium, selenium, and sulfur; a compound of cadmium, selenium, andtellurium; a compound of indium, gallium, and arsenic; a compound ofindium, gallium, and selenium; a compound of indium, selenium, andsulfur; a compound of copper, indium, and sulfur; and combinationsthereof. What is called an alloyed quantum dot, whose composition isrepresented by a given ratio, may be used.

Examples of the quantum dot include a core-type quantum dot, acore-shell quantum dot, and a core-multishell quantum dot. Quantum dotshave a high proportion of surface atoms and thus have high reactivityand easily cohere together. For this reason, it is preferable that aprotective agent be attached to, or a protective group be provided atthe surfaces of quantum dots. The attachment of the protective agent orthe provision of the protective group can prevent cohesion and increasesolubility in a solvent. It can also reduce reactivity and improveelectrical stability.

Since band gaps of quantum dots are increased as their size isdecreased, the size is adjusted as appropriate so that light with adesired wavelength can be obtained. Light emission from the quantum dotsis shifted to a blue color side, i.e., a high energy side, as thecrystal size is decreased; thus, emission wavelengths of the quantumdots can be adjusted over a wavelength range of a spectrum of anultraviolet region, a visible light region, and an infrared region bychanging the size of quantum dots. The range of size (diameter) ofquantum dots is, for example, greater than or equal to 0.5 nm and lessthan or equal to 20 nm, preferably greater than or equal to 1 nm andless than or equal to 10 nm. The emission spectra are narrowed as thesize distribution of the quantum dots gets smaller, and thus light canbe obtained with high color purity. The shape of the quantum dots is notparticularly limited and may be a spherical shape, a rod shape, acircular shape, or the like. A quantum rod, which is a rod-shapedquantum dot, has a function of emitting directional light.

The coloring layer is a colored layer that transmits light in a specificwavelength range. For example, a color filter for transmitting light ina red, green, blue, or yellow wavelength range can be used. Examples ofa material that can be used for the coloring layer include a metalmaterial, a resin material, and a resin material containing a pigment ora dye.

Through the above process, the display device 100A illustrated in FIG. 1can be manufactured.

FIG. 5A is a cross-sectional view of a display device 100B.

The display device 100B is different from the display device 100A inthat an insulating layer 188, an insulating layer 103, a light-blockinglayer BM, a coloring layer CFB, and a protective layer 126 are included.Other components are similar to those of the display device 100A.

In the display device 100B, the conductive layer 187 and the insulatinglayer 186 are provided over the insulating layer 185, and the insulatinglayer 188 over the conductive layer 187. Here, the top surface of theconductive layer 187 is level or substantially level with the topsurface of the insulating layer 186. For example, the top surfaces ofthe conductive layer 187 and the insulating layer 186 can be level witheach other by providing an opening in the insulating layer 186, formingthe conductive layer 187 to embed the opening, and performingplanarization treatment. An opening reaching the conductive layer 187 isprovided in the insulating layer 188, and the conductive layer 189 isprovided to fill the opening.

The display device 100B includes the insulating layer 103 between thelight-emitting diode 110 a and the color conversion layer CCR. As theinsulating layer 103, the above-described separation layer or aprotective layer of the light-emitting diodes 110 a and 110 b can beused, for example.

The light-blocking layer BM is preferably provided between adjacentpixels. FIG. 5A shows an example in which the light-blocking layer BM isprovided over the insulating layer 103, the color conversion layer CCRis provided in a position covering an end portion of the light-blockinglayer BM and overlapping with the light-emitting diode 110 a, and thecoloring layer CFB is provided in a position covering an end portion ofthe light-blocking layer BM and overlapping with the light-emittingdiode 110 b. The provision of the light-blocking layer BM can inhibit aphenomenon in which light emitted from the light-emitting diode 110 a isemitted to the outside of the display device 100B through the protectivelayer 102, the insulating layer 103, and the like without passingthrough the color conversion layer CCR and the coloring layer CFR.

In the display device 100B, the blue coloring layer CFB is provided overthe light-emitting diode 110 b. The purity of blue light emitted fromthe light-emitting diode 110 b in a blue pixel is improved by thecoloring layer CFB and the light is emitted to the outside of thedisplay device 100B.

The protective layer 126 may be provided to cover the color conversionlayer and the coloring layer. For the protective layer 126, resin suchas an acrylic resin, a polyimide resin, an epoxy resin, or a siliconeresin, and an inorganic insulating material such as silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, or aluminumoxide can be used. Furthermore, as the protective layer 126, a film suchas a resin film may be used.

FIG. 5B is a cross-sectional view of a display device 100C.

An example is shown in which the transistor 120 a and the transistor 120b of the display device 100C have different channel lengths. Othercomponents are similar to those of the display device 100A.

The transistor 120 a for driving the light-emitting diode 110 a and thetransistor 120 b for driving the light-emitting diode 110 b may bedifferent from each other in at least one of a transistor size, achannel length, a channel width, a structure, and the like.

Depending on the amount of current required for light emission withdesired luminance, one or both of the channel length and the channelwidth of the transistor may be changed between subpixels that emit lightof different colors.

FIG. 6 is a cross-sectional view of a display device 100D.

The display device 100D has a stacked-layer structure which includestransistors each including a channel formation region in a substrate 131(transistors 130 a and 130 b) and transistors each including a channelformation region in a metal oxide (the transistors 120 a and 120 b).

As the substrate 131, a single crystal silicon substrate is favorablyused. That is, the display device 100D preferably includes both atransistor including silicon in a channel formation region (hereinafter,also referred to as a Si transistor) and a transistor including a metaloxide in a channel formation region (hereinafter, also referred to as anOS transistor).

The transistors 130 a and 130 b each include a conductive layer 135, aninsulating layer 134, an insulating layer 136, and a pair oflow-resistance regions 133. The conductive layer 135 functions as agate. The insulating layer 134 is positioned between the conductivelayer 135 and the substrate 131 and functions as a gate insulatinglayer. The insulating layer 136 is provided to cover the side surface ofthe conductive layer 135 and functions as a sidewall. The pair oflow-resistance regions 133 in the substrate 131 is doped with animpurity; one of the pair of low-resistance regions 133 functions as asource of a transistor and the other functions as a drain of thetransistor.

An element isolation layer 132 is provided between two adjacenttransistors to be embedded in the substrate 131.

An insulating layer 139 is provided to cover the transistors 130 a and130 b, and a conductive layer 138 is provided over the insulating layer139. The conductive layer 138 is electrically connected to one of thepair of low-resistance regions 133 through a conductive layer 137provided to fill an opening in the insulating layer 139. That is, theconductive layer 137 functions as a plug for electrically connecting theconductive layer 138 and one of the pair of low-resistance regions 133.An insulating layer 141 is provided to cover the conductive layer 138,and a conductive layer 142 is provided over the insulating layer 141.The conductive layer 138 and the conductive layer 142 each function as awiring. An insulating layer 143 and the insulating layer 152 areprovided to cover the conductive layer 142, and the transistors 120 aand 120 b are provided over the insulating layer 152. A stacked-layerstructure including components from the insulating layer 152 to thecoloring layer CFR is the same as that of the display device 100A; thus,detailed description thereof is omitted.

The transistors 120 a, 120 b, 130 a, and 130 b can be used astransistors included in a pixel circuit or transistors included in adriver circuit (one or both of a gate driver and a source driver) fordriving the pixel circuit. The transistors 120 a, 120 b, 130 a, and 130b can also be used as transistors included in a variety of circuits suchas an arithmetic circuit and a memory circuit.

For example, the transistors 120 a and 120 b, which are OS transistors,can be used as transistors of the pixel circuit and the gate driver, andthe transistors 130 a and 130 b, which are Si transistors, can be usedas transistors of the source driver. Alternatively, the transistors 120a and 120 b, which are OS transistors, can be used as transistors of thepixel circuit and the gate driver, and the transistors 130 a and 130 b,which are Si transistors, can be used as transistors of the gate driverand the source driver.

With such a structure, not only the pixel circuit but also the drivercircuit or the like can be formed directly under the light-emittingdiode; thus, the display device can be downsized as compared with thecase where the driver circuit is provided outside a display portion. Inaddition, the display device can have a narrow frame (narrow non-displayregion).

As described above, the display device of one embodiment of the presentinvention can be manufactured by bonding a plurality of light-emittingdiodes and a plurality of transistors at a time, which leads to areduction in manufacturing cost of the display device and an improvementin yield. Moreover, by combining a micro LED and a transistor includinga metal oxide, a display device with reduced power consumption can beobtained.

[Structure Example 2 of Display Device]

FIG. 7A is a cross-sectional view of a display device 100E, and FIG. 7Bis a cross-sectional view of a display device 100F.

The display devices 100E and 100F are each manufactured by forming alight-emitting diode directly over a circuit board.

Here, it is difficult to form light-emitting diodes for different colorsover the same substrate because of a difference in a lattice constant.In the display device of this embodiment, full-color display isperformed by combination of light-emitting diodes of one kind and colorconversion layers. Therefore, only one kind of light-emitting diodes maybe formed over the substrate. Accordingly, the display device of thisembodiment can be manufactured by forming a light-emitting diodedirectly over a circuit board.

Components from the substrate 151 to the conductive layer 189 of thedisplay devices 100E and 100F are the same as those of the circuit board150B illustrated in FIG. 2B; thus, detailed description thereof isomitted.

An insulating layer 122 and the electrodes 112 and 116 of alight-emitting diode are provided over the conductive layer 189 and theinsulating layer 186.

For example, the electrode 112 of a light-emitting diode 110 c is incontact with and electrically connected to the conductive layer 189.Thus, the transistor 120 a and the light-emitting diode 110 c can beelectrically connected to each other. In the display devices 100E and100F, the electrode 112 and the electrode 116 function as a pixelelectrode and a common electrode of each of the light-emitting diode 110c and a light-emitting diode 110 d, respectively.

Here, the top surfaces of the electrodes 112 and 116 and the insulatinglayer 122 are level or substantially level with each other. For example,openings are provided in the insulating layer 122, the electrodes 112and 116 are formed to fill the openings, and planarization treatment isperformed, whereby the top surfaces of the electrodes 112 and 116 andthe insulating layer 122 can become level with each other. Accordingly,the semiconductor layer 113, the light-emitting layer 114, and thesemiconductor layer 115 can be formed over a flat surface.

The semiconductor layer 113 is provided over the electrode 112. Thesemiconductor layer 113 is electrically connected to the electrode 112.The light-emitting layer 114 is provided over the semiconductor layer113, and the semiconductor layer 115 is provided over the light-emittinglayer 114.

An insulating layer 123 is provided to cover the semiconductor layer113, the light-emitting layer 114, and the semiconductor layer 115. Anopening reaching the semiconductor layer 115 and an opening reaching theelectrode 116 are provided in the insulating layer 123, and conductivelayers 124 a and 124 b are provided to fill the openings.

A conductive layer 127 is provided over the conductive layers 124 a and124 b and the insulating layer 123. The conductive layer 124 a functionsas a plug for electrically connecting the semiconductor layer 115 andthe conductive layer 127. The conductive layer 124 b functions as a plugfor electrically connecting the conductive layer 127 and the electrode116. The semiconductor layer 115 is electrically connected to theelectrode 116 through the conductive layers 124 a, 127, and 124 b.

The light-blocking layer BM is preferably provided between adjacentpixels. FIGS. 7A and 7B show examples in which the light-blocking layerBM is provided over the insulating layer 123, the color conversion layerCCR is provided in a position covering an end portion of thelight-blocking layer BM and overlapping with the light-emitting diode110 c, and the red coloring layer CFR is provided over the colorconversion layer CCR. The provision of the light-blocking layer BM caninhibit a phenomenon in which light emitted from the light-emittingdiode 110 c is emitted to the outside of each of the display devices100E and 100F through the insulating layer 123 and the like withoutpassing through the color conversion layer CCR and the coloring layerCFR.

Note that an opening that surrounds the light-emitting diode may beprovided in the insulating layer 123 and the light-blocking layer BM maybe provided to fill the opening as illustrated in FIG. 7B. Thus, lightemitted from the light-emitting diode can be inhibited from reaching anadjacent pixel, which improves the display quality of the display device100F.

[Structure Example 3 of Display Device]

A structure and a manufacturing method of a display device 100G will bedescribed with reference to FIGS. 8A and 8B. Furthermore, a structureand a manufacturing method of a display device 100H will be describedwith reference to FIGS. 8A, 9A, and 9B.

The display devices 100G and 100H are each manufactured by forming acircuit including a transistor and the like directly over an LEDsubstrate.

As described above, in the display device of this embodiment, onelight-emitting diode and color conversion layers are combined to performfull-color display. Therefore, light-emitting diodes of all the pixelscan be formed over one substrate. Thus, a circuit including a transistorand the like is further formed directly over the substrate, whereby thedisplay device of this embodiment can be manufactured.

In manufacturing the display devices 100G and 100H, first, astacked-layer structure including components from the substrate 101 tothe conductive layer 187 is formed as illustrated in FIG. 8A.

The components from the substrate 101 to the protective layer 102 of thedisplay devices 100G and 100H are the same as those of the LED substrate150A illustrated in FIG. 2A; thus, detailed description thereof isomitted.

An insulating layer 104 is provided over the protective layer 102. Anopening reaching the semiconductor layer 113 and an opening reaching thesemiconductor layer 115 are provided in the insulating layer 104, andconductive layers 118 a and 118 b are provided to fill the openings.

Here, the top surfaces of the conductive layers 118 a and 118 b and theinsulating layer 104 are level or substantially level with each other.Accordingly, the electrodes 112 and 116 and also the transistors 120 aand 120 b and the like can be formed over a flat surface.

The electrodes 112 and 116 of the light-emitting diode and an insulatinglayer 106 are provided over the insulating layer 104. The conductivelayer 118 a functions as a plug for electrically connecting thesemiconductor layer 113 and the electrode 112. The conductive layer 118b functions as a plug for electrically connecting the semiconductorlayer 115 and the electrode 116.

In the display devices 100G and 100H, the electrode 116 functions as apixel electrode of the light-emitting diodes 110 a and 110 b. Theelectrode 112 functions as a common electrode of the light-emittingdiodes 110 a and 110 b.

The top surfaces of the electrodes 112 and 116 and the insulating layer106 are level or substantially level with each other. Accordingly, thetransistors 120 a and 120 b and the like can be formed over a flatsurface.

An insulating layer 108 and the insulating layer 152 are provided overthe insulating layer 106 and the electrodes 112 and 116. An openingreaching the electrode 116 is provided in the insulating layers 108 and152, and a plug for electrically connecting the electrode 116 and thetransistor is provided to fill the opening. The plug preferably includesa conductive layer 107 b in contact with the side surface of the openingand the top surface of the electrode 116 and a conductive layer 107 aembedded inside the conductive layer 107 b. Here, a conductive materialin which hydrogen and oxygen are less likely to be diffused ispreferably used for the conductive layer 107 b.

Components from the insulating layer 152 to the conductive layer 187 ofthe display devices 100G and 100H are the same as those of the circuitboard 150B illustrated in FIG. 2B except that conductive layers 161 a,184 c, and 184 d are included.

The conductive layer 161 a can be formed using the same material and inthe same step as those of the conductive layer 161 of the transistor.The conductive layer 184 c can be formed using the same material and inthe same step as the conductive layer 184 a. The conductive layer 184 dcan be formed using the same material and in the same step as theconductive layer 184 b.

An opening reaching the conductive layer 161 a is provided in theinsulating layers 163, 164, 181, 182, 183, and 185, and a plug forelectrically connecting the conductive layer 187 and the conductivelayer 161 a is provided to fill the opening. The plug includes theconductive layers 184 c and 184 d.

One of the pair of conductive layers 166 of the transistor iselectrically connected to the electrode 116 through the conductivelayers 184 a, 184 b, 187, 184 c, 184 d, 161 a, 107 a, and 107 b.

After the formation of the stacked-layer structure illustrated in FIG.8A, the color conversion layer CCR is formed in a position overlappingwith the light-emitting diode 110 a and the red coloring layer CFR isformed over the color conversion layer CCR as illustrated in FIG. 8B,whereby the display device 100G is manufactured.

Note that the insulating layer 186 may be provided over the insulatinglayer 183, and the color conversion layer CCR may be formed over theinsulating layer 186.

As illustrated in FIG. 8B, light emitted from the light-emitting diode110 a enters the color conversion layer CCR through the protective layer102 and a plurality of insulating layers, i.e., the insulating layers104 to 186, and is converted from blue light into red light by the colorconversion layer CCR, the purity of the red light is improved by acoloring layer CFR, and the red light is emitted to the outside of thedisplay device 100G.

As illustrated in FIG. 8B, blue light emitted from the light-emittingdiode 110 b is emitted to the outside of the display device 100G throughthe protective layer 102 and the plurality of insulating layers, i.e.,the insulating layers 104 to 186.

After the formation of the stacked-layer structure illustrated in FIG.8A, the substrate 101 is separated (FIG. 9A), and a substrate 191provided with the coloring layer CFR, a color conversion layer CCMR, andthe like is bonded to a surface exposed by the separation with the useof an adhesive layer 192, whereby the display device 100H illustrated inFIG. 9B can be manufactured.

As the adhesive layer 192, any of a variety of curable adhesives such asa reactive curable adhesive, a thermosetting adhesive, an anaerobicadhesive, and a photocurable adhesive such as an ultraviolet curableadhesive can be used. Alternatively, an adhesive sheet or the like maybe used.

Alternatively, the substrate 191 provided with the coloring layer CFR,the color conversion layer CCMR, and the like may be bonded to thesubstrate 101 with the use of the adhesive layer 192. In other words,the substrate 101 is not necessarily separated.

At this time, the substrate 101 is preferably thinned by polishing orthe like. This can increase the extraction efficiency of light emittedfrom the light-emitting diode. In addition, the display device can bethin and lightweight.

As illustrated in FIG. 9B, light emitted from the light-emitting diode110 a enters the color conversion layer CCR through the adhesive layer192, is converted from blue light into red light by the color conversionlayer CCR, the purity of the red light is improved by a coloring layerCFR, and the red light is emitted to the outside of the display device100H.

As illustrated in FIG. 9B, blue light emitted from the light-emittingdiode 110 b is emitted to the outside of the display device 100H throughthe adhesive layer 192.

As described above, in the display device of one embodiment of thepresent invention, a plurality of light-emitting diodes and a pluralityof transistors can be bonded to each other at a time. Alternatively, thedisplay device of one embodiment of the present invention can bemanufactured by forming a light-emitting diode directly over a circuitboard. Alternatively, the display device of one embodiment of thepresent invention can be manufactured by forming a transistor and thelike directly over a circuit board. Therefore, manufacturing cost can bereduced and a yield can be improved.

Moreover, by combining a micro LED and a transistor including a metaloxide, a display device with reduced power consumption can be obtained.

This embodiment can be combined with the other embodiments asappropriate. In this specification, in the case where a plurality ofstructure examples are shown in one embodiment, the structure examplescan be combined as appropriate.

Embodiment 2

In this embodiment, structure examples of transistors that can be usedfor the display device of one embodiment of the present invention aredescribed with reference to FIGS. 10A to 10D and FIGS. 11A to 11D.

The transistor of this embodiment can be small, which facilitates anincrease in the resolution of a display device and the application ofthe transistor to an electronic device having a relatively small displayportion.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. The transistorincludes a region where a channel is formed (a channel formation region)between the drain (a drain terminal, a drain region, or a drainelectrode) and the source (a source terminal, a source region, or asource electrode), and current can flow between the source and the drainthrough the channel formation region. Note that in this specificationand the like, a channel formation region refers to a region throughwhich current mainly flows.

Functions of a source and a drain might be switched when a transistor ofopposite polarity is employed or a direction of current flow is changedin circuit operation, for example. Therefore, the terms “source” and“drain” can be interchanged in some cases in this specification and thelike.

Note that the channel length refers to, for example, a distance betweena source (source region or source electrode) and a drain (drain regionor drain electrode) in a region where a semiconductor (or a portionwhere a current flows in a semiconductor when a transistor is on) and agate electrode overlap with each other or in a channel formation regionin a top view of the transistor. In one transistor, channel lengths inall regions are not necessarily the same. In other words, the channellength of one transistor is not fixed to one value in some cases.Therefore, in this specification, the channel length is any one ofvalues, the maximum value, the minimum value, or the average value in achannel formation region.

The channel width refers to, for example, the length of a channelformation region perpendicular to a channel length direction in a regionwhere a semiconductor (or a portion where a current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other or in the channel formation region in a top view of thetransistor. In one transistor, channel widths in all regions are notnecessarily the same. In other words, the channel width of onetransistor is not fixed to one value in some cases. Therefore, in thisspecification, the channel width is any one of values, the maximumvalue, the minimum value, or the average value in a channel formationregion.

Note that in this specification and the like, depending on thetransistor structure, a channel width in a region where a channel isactually formed (hereinafter also referred to as an effective channelwidth) is different from a channel width shown in a top view of atransistor (hereinafter also referred to as an apparent channel width)in some cases. For example, in a transistor having a gate electrodecovering the side surface of a semiconductor, the effective channelwidth is larger than the apparent channel width, and its influencecannot be ignored in some cases. As another example, in a miniaturizedtransistor having a gate electrode covering the side surface of asemiconductor, the proportion of a channel formation region formed onthe side surface of the semiconductor is sometimes increased. In thatcase, the effective channel width is larger than the apparent channelwidth.

In such cases, an effective channel width is sometimes difficult toestimate by measuring. For example, to estimate an effective channelwidth from a design value, it is necessary to assume that the shape of asemiconductor is known. Accordingly, in the case where the shape of asemiconductor is not known exactly, it is difficult to measure aneffective channel width accurately.

In this specification, the simple term “channel width” denotes anapparent channel width in some cases. In other cases, the simple term“channel width” denotes an effective channel width. Note that the valuesof a channel length, a channel width, an effective channel width, anapparent channel width, and the like can be determined by analyzing across-sectional image obtained by a transmission electron microscope(TEM) and the like.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of the semiconductor. Forexample, an element with a concentration lower than 0.1 atomic % isregarded as an impurity. When a semiconductor contains an impurity, anincrease in density of defect states or a reduction in crystallinity ofthe semiconductor may occur, for example. In the case where thesemiconductor is an oxide semiconductor, examples of an impurity thatchanges the characteristics of the semiconductor include Group 1elements, Group 2 elements, Group 13 elements, Group 14 elements, Group15 elements, and transition metals other than the main components of theoxide semiconductor. Specific examples include hydrogen, lithium,sodium, silicon, boron, phosphorus, carbon, and nitrogen. Note thatwater also serves as an impurity in some cases. Entry of an impurity maycause oxygen vacancies (Vo) in an oxide semiconductor, for example.

In this specification and the like, the term “insulator” can be replacedwith an insulating film or an insulating layer. The term “conductor” canbe replaced with a conductive film or a conductive layer. The term“semiconductor” can be replaced with a semiconductor film or asemiconductor layer.

In this specification and the like, a metal oxide means an oxide ofmetal in a broad sense. Metal oxides are classified into an oxideinsulator, an oxide conductor (including a transparent oxide conductor),an oxide semiconductor (also simply referred to as an OS), and the like.For example, a metal oxide used in a semiconductor layer of a transistoris referred to as an oxide semiconductor in some cases. That is, an OStransistor is a transistor including a metal oxide or an oxidesemiconductor.

FIGS. 10A to 10D illustrate a semiconductor device including atransistor 200A. FIG. 10A is a top view of the semiconductor device, andFIGS. 10B, 10C, and 10D are cross-sectional views taken alongdashed-dotted lines A1-A2, A3-A4, and A5-A6 illustrated in FIG. 10A,respectively. FIG. 10B can also be referred to as a cross-sectional viewof the transistor 200A in the channel length direction. FIG. 10C canalso be referred to as a cross-sectional view of the transistor 200A inthe channel width direction. Note that for simplification, somecomponents are not illustrated in FIG. 10A.

The semiconductor device illustrated in FIGS. 10A to 10D includes, aninsulator 212 over a substrate (not illustrated), an insulator 214 overthe insulator 212, an insulator 216 over the insulator 214, thetransistor 200A over the insulators 214 and 216, an insulator 254 overthe transistor 200A, an insulator 280 over the insulator 254, aninsulator 282 over the insulator 280, and an insulator 283 over theinsulator 282. The insulators 212, 214, 216, 254, 280, 282, and 283 eachfunction as an interlayer film. The semiconductor device furtherincludes a conductor 240 (a conductor 240 a and a conductor 240 b), aninsulator 241 (an insulator 241 a and an insulator 241 b), a conductor246 (a conductor 246 a and a conductor 246 b) over the insulator 283 andthe conductor 240, and an insulator 286 over the conductor 246 and theinsulator 283. The conductors 240 a and 240 b are electrically connectedto the transistor 200A and function as plugs. The insulator 241 isprovided in contact with the side surface of the conductor 240. Theconductor 246 a is electrically connected to the conductor 240 a andfunctions as a wiring. Similarly, the conductor 246 b is electricallyconnected to the conductor 240 b and functions as a wiring.

The insulator 241 a is provided in contact with a side wall of anopening formed in the insulators 254, 280, 282, and 283, a firstconductor of the conductor 240 a is provided in contact with the sidesurface of the insulator 241 a, and a second conductor of the conductor240 a is provided inside the first conductor. The insulator 241 b isprovided in contact with a side wall of an opening formed in theinsulators 254, 280, 282, and 283, a first conductor of the conductor240 b is provided in contact with the side surface of the insulator 241b, and a second conductor of the conductor 240 b is provided inside thefirst conductor. The top surface of the conductor 240 can besubstantially level with the top surface of the insulator 283 in aregion overlapping with the conductor 246. Although FIG. 10B illustratesa structure in which the conductor 240 has a stacked-layer structure ofthe first and second conductors, the conductor 240 may have asingle-layer structure or a stacked-layer structure. In the case where astacked-layer structure is employed in this specification and the like,the layers may be distinguished by numbers corresponding to theformation order.

The transistor 200A includes a conductor 205 (a conductor 205 a and aconductor 205 b) embedded in the insulator 216, an insulator 222 overthe insulator 216 and the conductor 205, an insulator 224 over theinsulator 222, an oxide 230 a over the insulator 224, an oxide 230 bover the oxide 230 a, an oxide 243 (an oxide 243 a and an oxide 243 b)and an oxide 230 c that are over the oxide 230 b, a conductor 242 a overthe oxide 243 a, a conductor 242 b over the oxide 243 b, an oxide 230 dover the oxide 230 c, an insulator 250 over the oxide 230 d, and aconductor 260 (a conductor 260 a and a conductor 260 b) that is over theinsulator 250 and overlaps with part of the oxide 230 c. The oxide 230 cis in contact with the side surfaces of the oxides 243 a and 243 b andthe conductors 242 a and 242 b. The insulator 282 is in contact with thetop surfaces of the conductor 260, the insulator 250, the oxide 230 d,the oxide 230 c, and the insulator 280.

An opening reaching the oxide 230 b is provided in the insulators 280and 254. The oxide 230 c, the oxide 230 d, the insulator 250, and theconductor 260 are provided in the opening. In addition, in the channellength direction of the transistor 200A, the conductor 260, theinsulator 250, the oxide 230 d, and the oxide 230 c are provided betweenthe conductor 242 a and the oxide 243 a and the conductor 242 b and theoxide 243 b. The insulator 250 includes a region in contact with theside surface of the conductor 260 and a region in contact with thebottom surface of the conductor 260. The oxide 230 c includes a regionin contact with the oxide 230 b, a region overlapping with the sidesurface of the conductor 260 with the oxide 230 d and the insulator 250therebetween, and a region overlapping with the bottom surface of theconductor 260 with the oxide 230 d and the insulator 250 therebetween.

The oxide 230 preferably includes the oxide 230 a over the insulator224, the oxide 230 b over the oxide 230 a, the oxide 230 c over and atleast partly in contact with the oxide 230 b, and the oxide 230 d overthe oxide 230 c.

Although the transistor 200A includes the oxide 230 with a four-layerstructure in which the oxide 230 a, the oxide 230 b, the oxide 230 c,and the oxide 230 d are stacked, the oxide 230 may have a single-layerstructure or a stacked-layer structure. For example, the oxide 230 maybe a single layer of the oxide 230 b or has a two-layer structure of theoxide 230 a and the oxide 230 b, a two-layer structure of the oxide 230b and the oxide 230 c, a three-layer structure of the oxide 230 a, theoxide 230 b, and the oxide 230 c, a three-layer structure of the oxide230 a, the oxide 230 b, and the oxide 230 d, or a stacked-layerstructure including five or more layers. Alternatively, each of theoxides 230 a, 230 b, 230 c, and 230 d may have a single-layer structureor a stacked-layer structure.

The conductor 260 functions as a first gate (top gate) electrode and theconductor 205 functions as a second gate (back gate) electrode. Theinsulators 250, 224, and 222 each function as a gate insulator. Theconductor 242 a functions as one of a source electrode and a drainelectrode, and the conductor 242 b functions as the other of the sourceelectrode and the drain electrode. The oxide 230 functions as a channelformation region.

In the transistor 200A, the oxide 230 (the oxides 230 a, 230 b, 230 c,and 230 d), which includes a channel formation region, is preferablyformed using a metal oxide functioning as a semiconductor (an oxidesemiconductor).

The band gap of the metal oxide functioning as a semiconductor ispreferably greater than or equal to 2.0 eV, further preferably greaterthan or equal to 2.5 eV. The use of a metal oxide having a wide band gapcan reduce the off-state current of the transistor.

A transistor including an oxide semiconductor in a channel formationregion has an extremely low leakage current (off-state current) in anoff state; thus, a semiconductor device with low power consumption canbe provided. In addition, an oxide semiconductor can be formed by asputtering method or the like and thus can be used for a transistorincluded in a highly integrated semiconductor device.

For example, as the oxide 230, a metal oxide such as an In-M-Zn oxidecontaining indium, an element M, and zinc is used; the element M is oneor more selected from aluminum, gallium, yttrium, tin, copper, vanadium,beryllium, boron, titanium, iron, nickel, germanium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, cobalt, and the like. Alternatively, an In—Ga oxide, an In—Znoxide, or indium oxide may be used as the oxide 230.

A transistor formed using an oxide semiconductor is likely to have itselectrical characteristics changed by impurities and oxygen vacancies(Vo) in a channel formation region in the oxide semiconductor; as aresult, the reliability is reduced, in some cases. Examples ofimpurities contained in an oxide semiconductor include hydrogen,nitrogen, alkali metal, alkaline earth metal, iron, nickel, and silicon.

In particular, hydrogen contained in an oxide semiconductor reacts withoxygen bonded to a metal atom, and thus causes H₂O and an oxygen vacancyin some cases. In some cases, a defect that is an oxygen vacancy intowhich hydrogen enters (also referred to as VoH) generates an electronserving as a carrier. Furthermore, some hydrogen may react with oxygenbonded to a metal atom and generate an electron serving as a carrier.Therefore, a transistor including an oxide semiconductor containinghydrogen easily has normally-on characteristics (characteristics suchthat a channel exists without voltage application to a gate electrodeand a current flows in a transistor).

A defect that is an oxygen vacancy into which hydrogen enters (VoH) canserve as a donor of the metal oxide. However, it is difficult toevaluate the defects quantitatively. Thus, in the metal oxide, thedefects are evaluated by carrier concentration, not by donorconcentration. Accordingly, in this specification and the like, carrierconcentration is sometimes used for a parameter of a metal oxide when anelectric field is not applied, instead of donor concentration. Hence,“carrier concentration” in this specification and the like can bereplaced with “donor concentration” in some cases. In addition, “carrierconcentration” in this specification and the like can be replaced with“carrier density”.

Accordingly, it is preferable that hydrogen and oxygen vacancies in thechannel formation region in the oxide semiconductor be reduced as muchas possible. Specifically, the hydrogen concentration in the oxide 230,which is measured by secondary ion mass spectrometry (SIMS), is lowerthan 1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, furtherpreferably lower than 5×10¹⁸ atoms/cm³, still further preferably lowerthan 1×10¹⁸ atoms/cm³. The oxide semiconductor preferably includes ani-type (intrinsic) or substantially i-type channel formation region witha low carrier concentration.

The oxide 230 preferably contains at least indium or zinc. Inparticular, indium and zinc are preferably contained. In addition,aluminum, gallium, yttrium, tin, or the like is preferably contained.Furthermore, one or more elements selected from boron, titanium, iron,nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium,hafnium, tantalum, tungsten, magnesium, cobalt, and the like may becontained.

Note that in this specification and the like, a metal oxide containingnitrogen is also referred to as a metal oxide in some cases. A metaloxide containing nitrogen may be referred to as a metal oxynitride.

Metal oxides that can be used as the oxide 230 will be described inEmbodiment 4 in detail.

The oxide 230 preferably has a stacked-layer structure of oxides withdifferent chemical compositions. The oxide 230 preferably has astacked-layer structure of oxides containing a common constituentelement (element serving as a main component) besides oxygen.

Specifically, the atomic ratio of the element M to In in the metal oxideused as the oxide 230 a or the oxide 230 d is preferably higher thanthat in the metal oxide used as the oxide 230 b or 230 c. The greaterthe atomic ratio of the element M to In is, the higher the capability ofinhibiting diffusion of impurities or oxygen is likely to be. Thus, whenthe oxide 230 a is provided under the oxide 230 b, impurities can beinhibited from being diffused into the oxide 230 b from the componentsformed below the oxide 230 a. Moreover, when the oxide 230 d is providedover the oxide 230 c, impurities can be inhibited from being diffusedinto the oxide 230 c from the components formed above the oxide 230 d.

In other words, the atomic ratio of In to the element M in the metaloxide used as the oxide 230 b or 230 c is preferably higher than that inthe metal oxide used as the oxide 230 a or the oxide 230 d. In thatcase, a main carrier path is the oxide 230 b, the oxide 230 c, or thevicinity thereof, for example, the interface between the oxides 230 band 230 c. The density of defect states at an interface between theoxides 230 b and 230 c can be made low when the oxide 230 b and theoxide 230 c contain a common constituent element (element serving as amain component) besides oxygen, whereby the influence of interfacescattering on carrier conduction is small and a high on-state currentcan be obtained.

In order to make the oxide 230 c serve as a main carrier path, theatomic ratio of indium to a metal element that is a main component inthe oxide 230 c is preferably higher than that in the oxide 230 b. Whena metal oxide having a high content of indium is used for a channelformation region, the on-state current of the transistor can beincreased.

Furthermore, in order to make the oxide 230 c serve as a main carrierpath, the conduction band minimum of the oxide 230 c is preferably moreapart from the vacuum level than those of the oxides 230 a, 230 b, and230 d. In other words, the electron affinity of the oxide 230 c ispreferably larger than those of the oxides 230 a, 230 b, and 230 d.

The oxides 230 b and 230 c preferably have crystallinity. In particular,for each of the oxides 230 b and 230 c, a c-axis-aligned crystallineoxide semiconductor (CAAC-OS) described later is preferably used. Theoxide 230 d may also have crystallinity.

The use of the CAAC-OS for each of the oxides 230 b and 230 c can reduceimpurities and oxygen vacancies in a channel formation region in theoxide semiconductor. Thus, the transistor can have high reliability andstable electrical characteristics with a small variation.

In addition, oxygen extraction from the oxide 230 b by the source ordrain electrode can be inhibited. This inhibits extraction of oxygenfrom the oxide 230 b even when heat treatment is performed; hence, thetransistor 200A is stable against high temperatures in the manufacturingprocess (i.e., thermal budget).

The CAAC-OS facilitates oxygen transfer in the direction perpendicularto the c-axis of a CAAC structure. Accordingly, oxygen contained in theoxide 230 c can be efficiently supplied to the oxide 230 b.

The CAAC-OS is a metal oxide having a dense structure with highcrystallinity and a low amount of impurities or defects (oxygenvacancies or the like). In particular, after a formation of a metaloxide, heat treatment is performed at a temperature at which the metaloxide does not become a polycrystal (e.g., 400° C. to 600° C.), wherebya CAAC-OS having a dense structure with higher crystallinity can beobtained. As the density of the CAAC-OS is increased in such a manner,diffusion of impurities or oxygen in the CAAC-OS can be reduced.

The oxide 230 includes a channel formation region of the transistor 200Aand a pair of low-resistance regions (a source region and a drainregion) provided to interpose the channel formation region. At leastpart of the channel formation region overlaps with the conductor 260.The conductor 242 (the conductor 242 a and the conductor 242 b) isprovided over the oxide 230 b, and a region having a lower resistancethan the channel formation region is formed in the vicinity of theconductor 242.

The source and drain regions have an increased carrier concentration anda lowered resistance because of having a low oxygen concentration orcontaining impurities such as hydrogen, nitrogen, and a metal element,for example. In other words, the source and drain regions have a highercarrier concentration and a lower resistance than the channel formationregion. The channel formation region has, for example, a higher oxygenconcentration or a lower impurity concentration than the source anddrain regions, and thus has a low carrier concentration and a highresistance.

In the oxide 230, the boundaries between the regions are difficult toclearly observe in some cases. The concentration of impurities such ashydrogen, nitrogen, and a metal element, which are detected in eachregion, may be gradually changed not only between the regions but alsoin each region. That is, the region closer to the channel formationregion preferably has a lower concentration of impurities such ashydrogen, nitrogen, and a metal element.

In order to increase oxygen concentration in the channel formationregion, an insulator containing oxygen that is released by heating (alsoreferred to as excess oxygen) is provided in the vicinity of the oxidesemiconductor so that the oxygen can be supplied from the insulator tothe oxide semiconductor by heat treatment. As a result, oxygen vacanciesin the channel formation region in the oxide semiconductor can be filledwith the supplied oxygen. Furthermore, the supplied oxygen reacts withhydrogen remaining in the oxide semiconductor, so that the hydrogen canbe removed as H₂O (dehydrogenation). Thus, formation of VoH in the oxidesemiconductor can be inhibited.

However, when an excess amount of oxygen is supplied to the sourceregion or the drain region, the carrier concentration in the sourceregion or the drain region is reduced, so that the on-state current orfield-effect mobility of the transistor 200A might be decreased.Furthermore, the distribution of oxygen supplied to the source region orthe drain region is uneven in the substrate plane, which leads tovariation in the characteristics of the semiconductor device includingthe transistor.

Therefore, the oxide semiconductor preferably includes an i-type(intrinsic) or substantially i-type channel formation region with a lowcarrier concentration and n-type source and drain regions with a highcarrier concentration. That is, it is preferable that oxygen be suppliedto the channel formation region in the oxide semiconductor and thesource and drain regions be not supplied with an excess amount ofoxygen.

The insulator 254 is formed by a sputtering method, whereby oxygen canbe implanted into the insulator 224, for example. Then, the oxygenimplanted into the insulator 224 is supplied to the oxide 230 b throughthe oxide 230 c. Accordingly, oxygen can be selectively supplied to theoxide 230 c and a region of the oxide 230 b that is in contact with theoxide 230 c, which account for a large portion of the channel formationregion.

Moreover, when the above CAAC-OS having a dense structure is used as theoxide 230 b, diffusion of impurities and oxygen in the oxide 230 b canbe reduced. Accordingly, the oxygen supplied to the channel formationregion in the oxide 230 b can be inhibited from being diffused into thesource region and the drain region in the oxide 230 b.

In such a manner, oxygen is selectively supplied to the channelformation region, so that the channel formation region can becomei-type, and oxygen is inhibited from being diffused into the sourceregion and the drain region, so that the source region and the drainregion can remain n-type. As a result, a change in the electricalcharacteristics of the transistor 200A can be inhibited, and thusvariation in the electrical characteristics of the transistors 200A inthe substrate plane can be inhibited.

The oxide 230 d preferably contains at least one of the metal elementscontained in the metal oxide used as the oxide 230 c, further preferablycontains all of these metal elements. For example, an In-M-Zn oxide, anIn—Zn oxide, or indium oxide is preferably used as the oxide 230 c, andan In-M-Zn oxide, an M-Zn oxide, or an oxide of the element M ispreferably used as the oxide 230 d. Accordingly, the density of defectstates at the interface of the oxide 230 c and the oxide 230 d can bereduced.

The oxide 230 d is preferably a metal oxide that inhibits diffusion ortransmission of much oxygen compared to the oxide 230 c. When the oxide230 d is provided between the insulator 250 and the oxide 230 c,diffusion of oxygen contained in the oxide 230 c or the insulator 280into the insulator 250 can be inhibited. Accordingly, the oxygen can beefficiently supplied to the oxide 230 b through the oxide 230 c.Furthermore, the conductor 260 can be prevented from being oxidized bythe oxygen supplied through the insulator 250.

When the atomic ratio of In to the metal element that is a maincomponent in the metal oxide used as the oxide 230 d is lower than thatin the metal oxide used as the oxide 230 c, diffusion of In toward theinsulator 250 can be inhibited. Since the insulator 250 functions as agate insulator, the transistor shows poor characteristics when In entersthe insulator 250 and the like. Thus, the oxide 230 d provided betweenthe oxide 230 c and the insulator 250 allows the semiconductor device tohave high reliability.

Here, the conduction band minimum is gradually varied at a junctionportion of each of the oxides 230 a, 230 b, 230 c, and 230 d. In otherwords, the conduction band minimum at the junction portion of each ofthe oxides 230 a, 230 b, 230 c, and 230 d is continuously varied orcontinuously connected. To achieve this, the density of defect states ina mixed layer formed at the interface between the oxides 230 a and 230b, the interface between the oxides 230 b and 230 c, and the interfacebetween the oxides 230 c and 230 d is decreased.

When the oxides 230 a and 230 b, the oxides 230 b and 230 c, or theoxides 230 c and 230 d contain the same element as a main component inaddition to oxygen, for example, a mixed layer with a low density ofdefect states can be formed. For example, in the case where the oxide230 b is an In-M-Zn oxide, an In-M-Zn oxide, an M-Zn oxide, an oxide ofthe element M, an In—Zn oxide, indium oxide, or the like may be used aseach of the oxides 230 a, 230 c, and 230 d.

Specifically, as the oxide 230 a, a metal oxide having an atomic ratioof In:M:Zn=1:3:4 or in the vicinity thereof, or In:M:Zn=1:1:0.5 or inthe vicinity thereof may be used. As the oxide 230 b, a metal oxidehaving an atomic ratio of In:M:Zn=1:1:1 or in the vicinity thereof orIn:M:Zn=4:2:3 or in the vicinity thereof may be used. As the oxide 230c, a metal oxide having an atomic ratio of In:M:Zn=4:2:3 or in thevicinity thereof, In:M:Zn=5:1:3 or in the vicinity thereof, orIn:M:Zn=10:1:3 or in the vicinity thereof or indium oxide may be used.As the oxide 230 d, a metal oxide having an atomic ratio ofIn:M:Zn=1:3:4 or in the vicinity thereof, M:Zn=2:1 or in the vicinitythereof, or M:Zn=2:5 or in the vicinity thereof or an oxide of theelement M may be used. Note that the vicinity of the atomic ratioincludes ±30% of an intended atomic ratio. Gallium is preferably used asthe element M.

When the oxides 230 a to 230 d have the above composition, the densityof defect states at the interface between the oxides 230 a and 230 b,the interface between the oxides 230 b and 230 c, and the interfacebetween the oxides 230 c and 230 d can be made low. This reduces theinfluence of interface scattering on carrier conduction, and thetransistor 200A can have a high on-state current and high frequencycharacteristics.

Furthermore, it is preferable that a groove be provided in the oxide 230b and the oxide 230 c including the CAAC-OS be embedded therein. At thistime, the oxide 230 c is provided to cover an inner wall (a side walland the bottom surface) of the groove.

The depth of the groove of the oxide 230 b is preferably the same orsubstantially the same as the thickness of the oxide 230 c. In otherwords, the top surface of the oxide 230 c in a region overlapping withthe oxide 230 b is preferably level or substantially level with theinterface between the oxides 230 b and 243. For example, when the bottomsurface of the insulator 222 is a reference, a difference between theheight of the interface between the oxides 230 b and 243 and the heightof the interface between the oxides 230 c and 230 d is preferablysmaller than or equal to the thickness of the oxide 230 c, furtherpreferably smaller than or equal to half of the thickness of the oxide230 c.

Such a structure reduces the effect of defects such as VoH andimpurities in the transistor, whereby a channel can be formed in theoxide 230 c. As a result, the transistor can obtain favorable electricalcharacteristics. Furthermore, a semiconductor device with less variationin transistor characteristics and high reliability can be provided.

Moreover, impurities at the interface between the oxides 230 b and 230 cand in the vicinity thereof are preferably reduced or removed. In thecase where the element M is not aluminum, it is particularly preferablethat impurities such as aluminum and silicon be reduced or removedbecause the impurities hinder an increase in the crystallinity or c-axisalignment of the oxides 230 c and 230 b. The concentration of aluminumatoms at the interface between the oxides 230 b and 230 c and in thevicinity thereof is preferably lower than or equal to 2.0 atomic %,further preferably lower than or equal to 1.5 atomic %, still furtherpreferably lower than or equal to 1.0 atomic %, for example.

Note that in a metal oxide, a region having become an amorphous-likeoxide semiconductor (a-like OS) where an increase in crystallinity orc-axis alignment is hindered by impurities such as aluminum and siliconis referred to as a non-CAAC region in some cases. In the non-CAACregion, a large amount of VoH is generated, so that it is highlypossible that the transistor is easily become normally on. Accordingly,the non-CAAC region is preferably reduced or removed.

In contrast, since a dense crystal structure is formed in the oxides 230b and 230 c having a CAAC structure, it is difficult for VoH to existstably. Furthermore, in oxygen adding treatment described later, excessoxygen is supplied to the oxides 230 b and 230 c, whereby VoH and Vo inthe oxides 230 b and 230 c can be reduced. When the oxides 230 b and 230c have a CAAC structure as described above, the transistor can beinhibited from becoming normally on.

As illustrated in FIG. 10C, a curved surface may be provided between theside and top surfaces of the oxide 230 b in a cross-sectional view inthe channel width direction of the transistor 200A. That is, an endportion of the side surface and an end portion of the top surface may becurved.

The radius of curvature of the curved surface is preferably greater than0 nm and less than the thickness of the oxide 230 b in a regionoverlapping with the conductor 242, or less than half of the length of aregion of the top surface of the oxide 230 b that does not have thecurved surface. Specifically, the radius of curvature of the curvedsurface is greater than 0 nm and less than or equal to 20 nm, preferablygreater than or equal to 1 nm and less than or equal to 15 nm, furtherpreferably greater than or equal to 2 nm and less than or equal to 10nm. Such a shape can improve the coverage of the groove with theinsulator 250 and the conductor 260, which are formed in a later step.Furthermore, reduction in the length of the region of the top surface ofthe oxide 230 b that does not have the curved surface can be prevented,and decrease in the on-state current and mobility of the transistor 200Acan be inhibited. Therefore, a semiconductor device having favorableelectrical characteristics can be provided.

Note that the oxide 230 c may be provided for each of the transistors200A. The oxides 230 c of two adjacent transistors 200A are notnecessarily in contact with each other. When the oxide 230 c is providedfor each of the transistors 200A, generation of a parasitic transistorbetween two transistors can be inhibited, which inhibits generation of aleakage path along the conductor 260. Thus, a semiconductor device thathas favorable electrical characteristics and can be miniaturized orhighly integrated can be provided.

Note that each of the conductor 260 and the insulator 250 may be sharedby adjacent transistors 200A. In other words, the conductor 260 of thetransistor 200A includes a region continuous with the conductor 260 ofthe adjacent transistor 200A. In addition, the insulator 250 of thetransistor 200A includes a region continuous with the insulator 250 ofthe adjacent transistor 200A.

In the above structure, the oxide 230 d includes a region in contactwith the insulator 224 between adjacent transistors 200A. Note that theoxide 230 d of the transistor 200A may be apart from the oxide 230 d ofthe adjacent transistor 200A. In that case, the insulator 250 includes aregion in contact with the insulator 224 between the transistor 200A andthe adjacent transistor 200A

At least one of the insulators 212, 214, 254, 282, 283, and 286preferably functions as a barrier insulating film that inhibitsdiffusion of impurities such as water and hydrogen into the transistor200A from a portion above the transistor 200A. Thus, the insulators 212,214, 254, 282, 283, and 286 are each preferably formed using aninsulating material having a function of inhibiting diffusion ofimpurities such as hydrogen atoms, hydrogen molecules, water molecules,nitrogen atoms, nitrogen molecules, nitrogen oxide molecules (e.g., N₂O,NO, and NO₂), and copper atoms, that is, an insulating material throughwhich the impurities are less likely to pass. Alternatively, theinsulators 212, 214, 254, 282, 283, and 286 are each preferably formedusing an insulating material having a function of inhibiting diffusionof oxygen (e.g., at least one of oxygen atoms and oxygen molecules),that is, an insulating material through which oxygen is less likely topass.

Note that in this specification, a barrier insulating film refers to aninsulating film having a barrier property. In this specification, abarrier property means a function of inhibiting diffusion of aparticular substance (or low permeability) or a function of capturing orfixing (also referred to as gettering) a particular substance.

For example, it is preferable that the insulators 212 and 283 be formedusing silicon nitride or the like, and the insulators 214, 254, and 282be formed using aluminum oxide or the like. Accordingly, impurities suchas water and hydrogen can be inhibited from diffusing to the transistor200A side from the substrate side through the insulators 212 and 214. Inaddition, oxygen contained in the insulator 224 and the like can beinhibited from diffusing to the substrate side through the insulators212 and 214. In this manner, the transistor 200A is preferablysurrounded by the insulators 212, 214, 254, 282, and 283 having afunction of inhibiting diffusion of oxygen and impurities such as waterand hydrogen.

The resistivity of the insulators 212, 283, and 286 is preferably low insome cases. For example, the insulators 212, 283, and 286 with aresistivity of approximately 1×10¹³ Ωcm can sometimes relieve chargebuildup of the conductor 205, the conductor 242, the conductor 260, orthe conductor 246 in the treatment using plasma or the like in themanufacturing process of a semiconductor device. The resistivity of theinsulators 212, 283, and 286 is preferably higher than or equal to1×10¹⁰ Ωcm and lower than or equal to 1×10¹⁵ Ωcm.

The insulators 216 and 280 preferably have a lower permittivity than theinsulator 214. The use of a material having a low dielectric constantfor the interlayer film can reduce the parasitic capacitance betweenwirings. For example, for the insulators 216 and 280, silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, siliconoxide to which fluorine is added, silicon oxide to which carbon isadded, silicon oxide to which carbon and nitrogen are added, poroussilicon oxide, or the like is used as appropriate. Silicon oxide andsilicon oxynitride are particularly preferable in terms of high thermalstability. Silicon oxide, silicon oxynitride, and porous silicon oxideare particularly preferable because a region containing oxygen that isreleased by heating can be easily formed in these materials.

The conductor 205 functions as a second gate electrode in some cases. Inthat case, by changing a potential applied to the conductor 205independently of a potential applied to the conductor 260, the thresholdvoltage (Vth) of the transistor 200A can be controlled. In particular,by applying a negative potential to the conductor 205, Vth of thetransistor 200A can be higher, and its off-state current can be reduced.This means that the conductor 205 with a negative potential can reducedrain current when 0 V is applied to the conductor 260, compared to theconductor 205 without a negative potential.

The conductor 205 is provided to overlap with the oxide 230 and theconductor 260.

As illustrated in FIG. 10A, the size of the conductor 205 is preferablylarger than the size of a region of the oxide 230 that does not overlapwith the conductors 242 a and 242 b. As illustrated in FIG. 10C, it isparticularly preferable that the conductor 205 extend beyond the endportions of the oxides 230 a and 230 b that intersect with the channelwidth direction. That is, the conductor 205 and the conductor 260preferably overlap with each other with the insulator positionedtherebetween, in a region beyond the side surface of the oxide 230 inthe channel width direction. With this structure, the channel formationregion in the oxide 230 can be electrically surrounded by electricfields of the conductor 260 functioning as the first gate electrode andelectric fields of the conductor 205 functioning as the second gateelectrode. In this specification, a transistor structure in which thechannel formation region is electrically surrounded by the electricfields of the first gate electrode and the second gate electrode isreferred to as a surrounded channel (S-channel) structure.

In this specification and the like, the S-channel structure refers to atransistor structure in which a channel formation region is electricallysurrounded by the electric fields of a pair of gate electrodes. TheS-channel structure disclosed in this specification and the like isdifferent from a Fin-type structure and a planar structure. With theS-channel structure, resistance to a short-channel effect can beenhanced, that is, a transistor in which a short-channel effect is lesslikely to occur can be provided.

As illustrated in FIG. 10C, the conductor 205 is extended to have afunction of a wiring. However, without limitation to this structure, aconductor functioning as a wiring may be provided under the conductor205. In addition, the conductor 205 is not necessarily provided in eachtransistor. For example, the conductor 205 may be shared by a pluralityof transistors.

Here, the conductor 205 a is preferably formed using a conductivematerial which has a function of inhibiting diffusion of impurities suchas hydrogen atoms, hydrogen molecules, water molecules, nitrogen atoms,nitrogen molecules, nitrogen oxide molecules (e.g., N₂O, NO, and NO₂),and copper atoms. Alternatively, the conductor 205 a is preferablyformed using a conductive material having a function of inhibitingdiffusion of oxygen (e.g., at least one of oxygen atoms and oxygenmolecules).

When a conductive material having a function of inhibiting oxygendiffusion is used for the conductor 205 a, a reduction in conductivityof the conductor 205 b due to oxidation of the conductor 205 b can beinhibited. As a conductive material having a function of inhibitingoxygen diffusion, tantalum, tantalum nitride, ruthenium, or rutheniumoxide is preferably used, for example. The conductor 205 a can thereforebe a single layer or a stack of the above conductive materials. Forexample, the conductor 205 a may be a stack of tantalum, tantalumnitride, ruthenium, or ruthenium oxide and titanium or titanium nitride.

A conductive material containing tungsten, copper, or aluminum as itsmain component is preferably used for the conductor 205 b. Note that theconductor 205 b is shown as a single layer but may have a stacked-layerstructure, for example, a stack of titanium or titanium nitride and theabove conductive material.

Although the transistor 200A in which the conductor 205 has astacked-layer structure of the conductors 205 a and 205 b isillustrated, the conductor 205 may have a single-layer structure or astacked-layer structure.

The insulator 222 preferably has a function of inhibiting diffusion ofhydrogen (e.g., at least one of hydrogen atoms, hydrogen molecules, andthe like). Moreover, the insulator 222 preferably has a function ofinhibiting diffusion of oxygen (e.g., at least one of oxygen atoms,oxygen molecules, and the like). For example, the insulator 222preferably has a function of inhibiting diffusion of much hydrogenand/or oxygen compared to the insulator 224.

As the insulator 222, an insulator containing an oxide of one or both ofaluminum and hafnium, which are insulating materials, is preferablyused. The insulator containing an oxide of one or both of aluminum andhafnium has a barrier property against oxygen, hydrogen, and water. Asthe insulator, aluminum oxide, hafnium oxide, an oxide containingaluminum and hafnium (hafnium aluminate), or the like is preferablyused. The insulator 222 formed of such a material functions as a layerthat inhibits release of oxygen from the oxide 230 to the substrate sideand diffusion of impurities such as hydrogen from the periphery of thetransistor 200A into the oxide 230. Thus, providing the insulator 222can inhibit diffusion of impurities such as hydrogen into the transistor200A and inhibit generation of oxygen vacancies in the oxide 230. Theinsulator 222 can also inhibit oxidization of the conductor 205 withoxygen contained in the insulator 224 and the oxide 230.

Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobiumoxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, orzirconium oxide may be added to the above insulator, for example.Alternatively, the insulator may be subjected to nitriding treatment.The insulator 222 may have a stacked-layer structure including siliconoxide, silicon oxynitride, or silicon nitride over any of theseinsulators.

The insulator 222 may have, for example, a single-layer structure or astacked-layer structure using an insulator containing a so-called high-kmaterial, such as aluminum oxide, hafnium oxide, tantalum oxide,zirconium oxide, lead zirconate titanate (PZT), strontium titanate(SrTiO₃), or (Ba,Sr)TiO₃ (BST). With miniaturization and highintegration of transistors, a problem such as generation of leakagecurrent may arise because of a thinner gate insulator. When a high-kmaterial is used for an insulator functioning as the gate insulator, agate potential at the time of operating the transistor can be reducedwhile the physical thickness of the gate insulator is kept.

The insulator 224 in contact with the oxide 230 preferably releasesoxygen by heating. For example, silicon oxide or silicon oxynitride canbe used as appropriate for the insulator 224. When an insulatorcontaining oxygen is provided in contact with the oxide 230, oxygenvacancies in the oxide 230 can be reduced, leading to an improvement inreliability of the transistor 200A.

For the insulator 224, it is preferable to use an insulating materialhaving a region containing oxygen in excess of that in thestoichiometric composition (hereinafter also referred to as anexcess-oxygen region). An oxide film including an excess oxygen regionor excess oxygen is an oxide film in which the amount of released oxygenmolecules is greater than or equal to 1.0×10¹⁸ molecules/cm³, preferablygreater than or equal to 1.0×10¹⁹ molecules/cm³, further preferablygreater than or equal to 2.0×10¹⁹ molecules/cm³ or greater than or equalto 3.0×10²⁰ molecules/cm³ in thermal desorption spectroscopy (TDS)analysis. Note that the temperature of the film surface in the TDSanalysis is preferably higher than or equal to 100° C. and lower than orequal to 700° C., or higher than or equal to 100° C. and lower than orequal to 400° C.

One or more of heat treatment, microwave treatment, and RF treatment maybe performed in a state in which the insulator including anexcess-oxygen region and the oxide 230 are in contact with each other.By the treatment, water or hydrogen in the oxide 230 can be removed.Some hydrogen is diffused in or trapped (also referred to as gettering)by the conductor 242 in some cases.

For the microwave treatment, for example, an apparatus having a powersupply that generates high-density plasma or an apparatus having a powersupply that applies RF to the substrate side is suitably used. Forexample, high-density oxygen radicals can be generated with the use ofan oxygen-containing gas and high-density plasma, and by applying RF tothe substrate side, the oxygen radicals generated by high-density plasmacan be introduced into the oxide 230 or the insulator near the oxide 230efficiently. The pressure in the microwave treatment is higher than orequal to 133 Pa, preferably higher than or equal to 200 Pa, furtherpreferably higher than or equal to 400 Pa. As a gas introduced into anapparatus in which the microwave treatment is performed, for example,oxygen and argon are used and the oxygen flow rate (O₂/(O₂+Ar)) is lessthan or equal to 50%, preferably greater than or equal to 10% and lessthan or equal to 30%.

Note that the oxygen adding treatment performed on the oxide 230 canpromote a reaction in which oxygen vacancies in the oxide 230 are filledwith supplied oxygen. Furthermore, hydrogen remaining in the oxide 230reacts with supplied oxygen, so that the hydrogen can be removed as H₂O(dehydrogenation). This can inhibit recombination of hydrogen remainingin the oxide 230 with oxygen vacancies and formation of VoH.

Note that the insulators 222 and 224 may each have a stacked-layerstructure of two or more layers. In that case, the stacked layers arenot necessarily formed of the same material and may be formed ofdifferent materials.

The oxide 243 preferably has a function of inhibiting oxygentransmission. When the oxide 243, which has a function of inhibitingoxygen transmission, is provided between the conductor 242, functioningas the source electrode or the drain electrode, and the oxide 230 b, theelectrical resistance between the conductor 242 and the oxide 230 b canbe reduced, which is preferable. Such a structure improves theelectrical characteristics and reliability of the transistor 200A. Inthe case where the electrical resistance between the conductor 242 andthe oxide 230 b can be sufficiently reduced, the oxide 243 is notnecessarily provided.

A metal oxide containing the element M may be used as the oxide 243. Inparticular, aluminum, gallium, yttrium, or tin is preferably used as theelement M The concentration of the element Min the oxide 243 ispreferably higher than that in the oxide 230 b. Alternatively, galliumoxide may be used as the oxide 243. A metal oxide such as an In-M-Znoxide may be used as the oxide 243. Specifically, the atomic ratio ofthe element M to In in the metal oxide used as the oxide 243 ispreferably higher than that in the metal oxide used as the oxide 230 b.The thickness of the oxide 243 ranges preferably from 0.5 nm to 5 nm,further preferably from 1 nm to 3 nm, still further preferably from 1 nmto 2 nm. The oxide 243 preferably has crystallinity. The oxide 243 withcrystallinity efficiently inhibits release of oxygen from the oxide 230.When the oxide 243 has a hexagonal crystal structure, for example,release of oxygen from the oxide 230 can sometimes be inhibited.

For the conductor 242, for example, a nitride containing tantalum, anitride containing titanium, a nitride containing molybdenum, a nitridecontaining tungsten, a nitride containing tantalum and aluminum, anitride containing titanium and aluminum, or the like is preferablyused. In this embodiment, a nitride containing tantalum is particularlypreferable. As another example, ruthenium oxide, ruthenium nitride,oxide containing strontium and ruthenium, or oxide containing lanthanumand nickel may be used. These materials are preferable because they area conductive material that is not easily oxidized or a material thatmaintains the conductivity even when absorbing oxygen.

There is a curved surface between the side and top surfaces of theconductor 242 in some cases. That is, the end portion of the sidesurface and the end portion of the top surface are curved in some cases.A radius of curvature of the curved surface at the end portion of theconductor 242 is greater than or equal to 3 nm and less than or equal to10 nm, preferably greater than or equal to 5 nm and less than or equalto 6 nm. When the end portion is not angular, the coverage with filmsformed in the following film formation steps is improved.

When the oxide 243 is not provided, contact between the conductor 242and the oxide 230 b or 230 c may make oxygen in the oxide 230 b or 230 cdiffuse into the conductor 242, resulting in oxidation of the conductor242. It is highly possible that oxidation of the conductor 242 lowersthe conductivity of the conductor 242. Note that diffusion of oxygenfrom the oxide 230 b or 230 c into the conductor 242 can be interpretedas absorption of oxygen in the oxide 230 b or 230 c by the conductor242.

When oxygen in the oxide 230 b or 230 c is diffused into the conductors242 a and 242 b, layers are sometimes formed between the conductor 242 aand the oxide 230 b and between the conductor 242 b and the oxide 230 bor between the conductor 242 a and the oxide 230 c and between theconductor 242 b and the oxide 230 c. The layers contain more oxygen thanthe conductor 242 a or 242 b, so that the layers presumably have aninsulating property. The three-layer structure of the conductor 242 a or242 b, the layer, and the oxide 230 b or 230 c can be the structure witha metal, an insulator, and a semiconductor, which can be regarded as ametal-insulator-semiconductor (MIS) structure or a diode junctionstructure having an MIS structure as its main part.

Note that hydrogen contained in the oxide 230 b, the oxide 230 c, or thelike is diffused into the conductor 242 a or 242 b in some cases. Inparticular, when a nitride containing tantalum is used for theconductors 242 a and 242 b, hydrogen contained in the oxide 230 b, theoxide 230 c, or the like is likely to be diffused into the conductor 242a or 242 b, and the diffused hydrogen is bonded to nitrogen contained inthe conductor 242 a or 242 b in some cases. That is, hydrogen containedin the oxide 230 b, the oxide 230 c, or the like is sometimes absorbedby the conductor 242 a or 242 b in some cases.

The insulator 254 is provided to cover the side surfaces of the oxides230 a, 230 b, and 243 and the top and side surfaces of the conductor242.

Thus, the insulator 254 preferably has a function of inhibiting oxygendiffusion. For example, it is preferable that the insulator 254 be morelikely to inhibit diffusion of oxygen than the insulator 280. As theinsulator 254, an insulator containing an oxide of one or both ofaluminum and hafnium is formed, for example.

As the insulator 254, aluminum oxide or hafnium oxide is preferablyformed in an oxygen-containing atmosphere by a bias sputtering method.The bias sputtering method is a method in which sputtering is performedwhile RF power is applied to a substrate. The potential of the substrateapplied with the RF power becomes a negative potential (bias potential)with respect to a plasma potential, and cations in plasma areaccelerated by the bias potential and implanted into the substrate. Thebias potential can be controlled by the amount of RF power applied tothe substrate. Therefore, aluminum oxide or hafnium oxide is formed bythe bias sputtering method in an oxygen-containing atmosphere, wherebyoxygen can be implanted into the insulator 224.

In the bias sputtering method, the amount of oxygen implanted into theinsulator 224 serving as a base of the insulator 254 can be controlledby the amount RF power applied to the substrate. For example, as the RFpower, a bias with the power density of 0.31 W/cm² or more, preferably0.62 W/cm² or more, further preferably 1.86 W/cm² or more is applied tothe substrate. In other words, an appropriate amount of oxygen for thetransistor characteristics can be implanted by changing the amount of RFpower used for the formation of the insulator 254. Moreover, anappropriate amount of oxygen for improving the reliability of thetransistor can be implanted. The RF frequency is preferably 10 MHz orhigher, typically 13.56 MHz. The higher the RF frequency is, the lessdamage to the substrate can be. Therefore, the amount of oxygen to beimplanted into the insulator 224 can be controlled by adjusting theamount of RF power applied to the substrate, so that the optimal amountof oxygen can be implanted into the insulator 224.

Note that a bias applied to the substrate in the bias sputtering methodis not limited to the RF power and may be a DC voltage.

As described above, although oxygen is implanted into the film servingas a base in the formation process of the insulator 254, the insulator254 itself has a function of inhibiting oxygen transmission.Accordingly, when the insulator 280 is formed over the insulator 254 andoxygen is diffused from the insulator 280 in a later step, the oxygencan be prevented from being directly diffused from the insulator 280into the oxides 230 a, 230 b, and 243 and the conductor 242.

With such an insulator 254, the oxides 230 a, 230 b, and 243 and theconductor 242 can be spaced from the insulator 280. Thus, oxygen can beprevented from being directly diffused from the insulator 280 into theoxides 230 a, 230 b, and 243 and the conductor 242. Accordingly, areduction in the carrier concentration in the source and drain regionsof the oxide 230 due to supply of excess oxygen to the source and drainregions can be prevented. Furthermore, the conductor 242 can beinhibited from being excessively oxidized to have a high resistivity,and thus a reduction in an on-state current can be inhibited.

The insulator 250 is preferably in contact with at least part of theoxide 230 d. For example, for the insulator 250, silicon oxide, siliconoxynitride, silicon nitride oxide, silicon nitride, silicon oxide towhich fluorine is added, silicon oxide to which carbon is added, siliconoxide to which carbon and nitrogen are added, porous silicon oxide, orthe like can be used. Silicon oxide and silicon oxynitride areparticularly preferable in terms of high thermal stability.

Like the insulator 224, the insulator 250 is preferably formed using aninsulator from which oxygen is released by heating. When the insulatorfrom which oxygen is released by heating is provided as the insulator250 to be in contact with at least part of the oxide 230 d, oxygen canbe effectively supplied to the channel formation region in the oxide 230and oxygen vacancies in the channel formation region in the oxide 230can be reduced. Thus, the transistor can have high reliability andstable electrical characteristics with a small variation. Furthermore,as in the insulator 224, the concentration of impurities such as waterand hydrogen in the insulator 250 is preferably lowered. The thicknessof the insulator 250 is preferably greater than or equal to 1 nm andless than or equal to 20 nm.

Although the insulator 250 is a single layer in FIGS. 10B and 10C, astacked-layer structure of two or more layers may be employed. In thecase where the insulator 250 has a stacked-layer structure including twolayers, it is preferable that a lower layer of the insulator 250 beformed using an insulator from which oxygen is released by heating andan upper layer of the insulator 250 be formed using an insulator havinga function of inhibiting diffusion of oxygen. Owing to this structure,diffusion of oxygen contained in the lower layer of the insulator 250into the conductor 260 can be inhibited. That is, a reduction in theamount of oxygen supplied to the oxide 230 can be inhibited. Moreover,oxidation of the conductor 260 due to oxygen contained in the lowerlayer of the insulator 250 can be inhibited. For example, the lowerlayer of the insulator 250 can be formed using the above-describedmaterial that can be used for the insulator 250, and the upper layer ofthe insulator 250 can be formed using a material similar to that for theinsulator 222.

In the case where silicon oxide, silicon oxynitride, or the like is usedfor the lower layer of the insulator 250, the upper layer of theinsulator 250 may be formed using an insulating material that is ahigh-k material having a high relative dielectric constant. The gateinsulator having a stacked-layer structure of the lower layer of theinsulator 250 and the upper layer of the insulator 250 can be thermallystable and can have a high dielectric constant. Accordingly, a gatepotential applied during operation of the transistor can be reducedwhile the physical thickness of the gate insulator is kept. In addition,the equivalent oxide thickness (EOT) of the insulator functioning as thegate insulator can be reduced.

Specifically, for the upper layer of the insulator 250, a metal oxidecontaining one or more of hafnium, aluminum, gallium, yttrium,zirconium, tungsten, titanium, tantalum, nickel, germanium, magnesium,and the like or a metal oxide that can be used as the oxide 230 can beused. In particular, an insulator containing an oxide of one or both ofaluminum and hafnium is preferably used.

Since the distance between the conductor 260 and the oxide 230 is keptby the physical thickness of the insulator 250 having a stacked-layerstructure including two layers, leakage current between the conductor260 and the oxide 230 can be reduced. Moreover, with the stacked-layerstructure of the insulator 250, the physical distance between theconductor 260 and the oxide 230 and the intensity of electric fieldsapplied from the conductor 260 to the oxide 230 can be easily adjustedas appropriate.

A metal oxide may be provided between the insulator 250 and theconductor 260. The metal oxide preferably prevents oxygen diffusion fromthe insulator 250 into the conductor 260. Providing the metal oxide thatinhibits oxygen diffusion inhibits diffusion of oxygen from theinsulator 250 into the conductor 260. That is, a reduction in the amountof oxygen supplied to the oxide 230 can be inhibited. Moreover,oxidation of the conductor 260 due to oxygen in the insulator 250 can beinhibited.

Note that the metal oxide preferably has a function of part of the firstgate electrode. With the metal oxide, the on-state current of thetransistor 200A can be increased without a reduction in the influence ofthe electric field applied from the conductor 260. For example, a metaloxide that can be used as the oxide 230 can be used as the metal oxide.In that case, when the conductor 260 a is formed by a sputtering method,the metal oxide can have a reduced electrical resistance and become aconductor. Such a conductor can be referred to as an oxide conductor(OC) electrode.

The conductor 260 preferably includes the conductor 260 a and theconductor 260 b over the conductor 260 a. For example, the conductor 260a is preferably positioned so as to cover the bottom and side surfacesof the conductor 260 b. Note that as illustrated in FIGS. 10B and 10C,the top surface of the conductor 260 is substantially level with the topsurfaces of the insulator 250, the oxide 230 d, and the oxide 230 c.Although FIGS. 10B and 10C show that the conductor 260 has a two-layerstructure of the conductor 260 a and the conductor 260 b, the conductor260 may have a single-layer structure or a stacked-layer structure ofthree or more layers.

Like the conductor 205 a, the conductor 260 a is preferably formed usinga conductive material having a function of inhibiting diffusion ofimpurities. Alternatively, the conductor 260 a is preferably formedusing a conductive material having a function of inhibiting diffusion ofoxygen.

When the conductor 260 a has a function of inhibiting diffusion ofoxygen, the conductivity of the conductor 260 b can be prevented frombeing lowered because of oxidization of the conductor 260 b due tooxygen in the insulator 250. As a conductive material having a functionof inhibiting oxygen diffusion, tantalum, tantalum nitride, ruthenium,or ruthenium oxide is preferably used, for example.

The conductor 260 also functions as a wiring and thus is preferably aconductor having high conductivity. For example, a conductive materialcontaining tungsten, copper, or aluminum as its main component can beused for the conductor 260 b. The conductor 260 b may have astacked-layer structure, for example, a stacked-layer structure oftitanium or titanium nitride and the above conductive material.

In the transistor 200A, the conductor 260 is formed in a self-alignedmanner so as to fill an opening formed in the insulator 280 and thelike. In this manner, the conductor 260 can surely be provided in aregion between the conductor 242 a and the conductor 242 b withoutalignment.

As illustrated in FIG. 10C, in the channel width direction of thetransistor 200A, the bottom surface of the conductor 260 in a regionthat does not overlap with the oxide 230 b is preferably lower in levelthan the bottom surface of the oxide 230 b. When the conductor 260functioning as the gate electrode covers the side and top surfaces ofthe channel formation region in the oxide 230 b with the insulator 250and the like therebetween, the electric field of the conductor 260 islikely to affect the entire channel formation region in the oxide 230 b.Hence, the transistor 200A can have a higher on-state current and higherfrequency characteristics. With the level of the bottom surface of theinsulator 222 as a reference, a distance between the bottom surface ofthe conductor 260 and the bottom surfaces of the oxide 230 b in a regionwhere the conductor 260 does not overlap with the oxides 230 a and 230 bis greater than or equal to 0 nm and less than or equal to 100 nm,preferably greater than or equal to 3 nm and less than or equal to 50nm, and further preferably greater than or equal to 5 nm and less thanor equal to 20 nm.

The insulator 280 is provided over the insulator 254. The top surface ofthe insulator 280 may be planarized.

The concentration of impurities such as water and hydrogen in theinsulator 280 is preferably reduced. Moreover, the insulator 280preferably has a low hydrogen concentration and includes anexcess-oxygen region or excess oxygen, and may be formed using amaterial similar to that for the insulator 216, for example. Theinsulator 280 may have a stacked-layer structure of the above materials;silicon oxide formed by a sputtering method and silicon oxynitrideformed by a chemical vapor deposition (CVD) method thereover. Siliconnitride can be stacked over the above stacked structure.

The conductor 240 is preferably formed using a conductive materialcontaining tungsten, copper, or aluminum as its main component. Theconductor 240 may have a stacked-layer structure. In the case where theconductor 240 has a stacked-layer structure, a conductive materialhaving a function of inhibiting transmission of impurities such as waterand hydrogen is preferably used as a conductor in contact with theinsulators 283, 282, 280, and 254. For example, tantalum, tantalumnitride, titanium, titanium nitride, ruthenium, or ruthenium oxide ispreferably used. The conductive material having a function of inhibitingtransmission of impurities such as water and hydrogen can be used as asingle layer or stacked layers. Furthermore, impurities such as waterand hydrogen contained in the components above the insulator 283 can beprevented from entering the oxide 230 through the conductors 240 a and240 b.

An insulator such as silicon nitride, aluminum oxide, or silicon nitrideoxide may be used as the insulators 241 a and 241 b, for example. Sincethe insulators 241 a and 241 b are provided in contact with theinsulator 254, impurities such as water and hydrogen contained in theinsulator 280 or the like can be prevented from entering the oxide 230through the conductors 240 a and 240 b. Silicon nitride is particularlypreferable because of its high blocking property against hydrogen.Furthermore, oxygen contained in the insulator 280 can be prevented frombeing absorbed by the conductors 240 a and 240 b.

The conductor 246 (the conductors 246 a and 246 b) functioning as awiring may be provided in contact with the top surfaces of theconductors 240 a and 240 b. The conductor 246 is preferably formed usinga conductive material containing tungsten, copper, or aluminum as itsmain component. The conductor may have a stacked-layer structure, forexample, a stack of titanium or titanium nitride and the aboveconductive material. Note that the conductor may be formed to beprovided to fill an opening in an insulator.

The insulator 286 is provided over the conductor 246 and the insulator283. Accordingly, the top and side surfaces of the conductor 246 are incontact with the insulator 286, and the bottom surface of the conductor246 is in contact with the insulator 283. In other words, the conductor246 can be surrounded by the insulators 283 and 286. The structure caninhibit transmission of oxygen from the outside and oxidation of theconductor 246. Furthermore, this can inhibit diffusion of impuritiessuch as water and hydrogen from the conductor 246 to the outside, whichis preferable.

As a substrate where the transistor 200A is formed, an insulatorsubstrate, a semiconductor substrate, or a conductor substrate can beused, for example. Examples of the insulator substrate include a glasssubstrate, a quartz substrate, a sapphire substrate, a stabilizedzirconia substrate (e.g., an yttria-stabilized zirconia substrate), anda resin substrate. Examples of the semiconductor substrate include asemiconductor substrate of silicon or germanium and a compoundsemiconductor substrate of silicon carbide, silicon germanium, galliumarsenide, indium phosphide, zinc oxide, or gallium oxide. Other examplesinclude a semiconductor substrate in which an insulator region isprovided in the above semiconductor substrate such as an SOI substrate.Examples of the conductor substrate include a graphite substrate, ametal substrate, an alloy substrate, and a conductive resin substrate. Asubstrate containing a nitride of a metal, a substrate including anoxide of a metal, or the like can also be used. Moreover, an insulatorsubstrate provided with a conductor or a semiconductor, a semiconductorsubstrate provided with a conductor or an insulator, a conductorsubstrate provided with a semiconductor or an insulator, or the like maybe used. Alternatively, any of these substrates provided with an elementmay be used. Examples of the element provided over the substrate includea capacitor, a resistor, a switching element, a light-emitting element,and a memory element.

Examples of an insulator included in the semiconductor device include aninsulating oxide, an insulating nitride, an insulating oxynitride, aninsulating nitride oxide, an insulating metal oxide, an insulating metaloxynitride, and an insulating metal nitride oxide.

With miniaturization and high integration of transistors, for example, aproblem such as generation of leakage current or the like may arisebecause of a thinner gate insulator. When a high-k material is used foran insulator functioning as a gate insulator, the driving voltage of thetransistor can be reduced while the physical thickness of the gateinsulator is kept. On the other hand, when a material having a lowdielectric constant is used for an insulator functioning as aninterlayer film, the parasitic capacitance between wirings can bereduced. A material is preferably selected depending on the function ofan insulator.

Examples of the insulator having a high dielectric constant includegallium oxide, hafnium oxide, zirconium oxide, an oxide containingaluminum and hafnium, an oxynitride containing aluminum and hafnium, anoxide containing silicon and hafnium, an oxynitride containing siliconand hafnium, and a nitride containing silicon and hafnium.

Examples of the insulator having a low dielectric constant includesilicon oxide, silicon oxynitride, silicon nitride oxide, siliconnitride, silicon oxide to which fluorine is added, silicon oxide towhich carbon is added, silicon oxide to which carbon and nitrogen areadded, porous silicon oxide, and a resin.

A transistor including a metal oxide can have stable electricalcharacteristics when surrounded by an insulator having a function ofinhibiting transmission of oxygen and impurities such as hydrogen. Theinsulator having a function of inhibiting transmission of oxygen andimpurities such as hydrogen can have, for example, a single-layerstructure or a stacked-layer structure of an insulator containing boron,carbon, nitrogen, oxygen, fluorine, magnesium, aluminum, silicon,phosphorus, chlorine, argon, gallium, germanium, yttrium, zirconium,lanthanum, neodymium, hafnium, or tantalum. Specifically, as theinsulator having a function of inhibiting transmission of oxygen andimpurities such as hydrogen, a metal oxide such as aluminum oxide,magnesium oxide, gallium oxide, germanium oxide, yttrium oxide,zirconium oxide, lanthanum oxide, neodymium oxide, hafnium oxide, ortantalum oxide or a metal nitride such as aluminum nitride, siliconnitride oxide, or silicon nitride can be used.

An insulator functioning as a gate insulator preferably includes aregion containing oxygen that is released by heating. For example,silicon oxide or silicon oxynitride that includes a region containingoxygen that is released by heating is provided in contact with the oxide230 to compensate for the oxygen vacancies in the oxide 230.

For the conductor included in the semiconductor device, it is preferableto use a metal element selected from aluminum, chromium, copper, silver,gold, platinum, tantalum, nickel, titanium, molybdenum, tungsten,hafnium, vanadium, niobium, manganese, magnesium, zirconium, beryllium,indium, ruthenium, iridium, strontium, lanthanum, and the like; an alloycontaining any of the above metal elements; an alloy containing acombination of the above metal elements; or the like. For example,tantalum nitride, titanium nitride, tungsten, a nitride containingtitanium and aluminum, a nitride containing tantalum and aluminum,ruthenium oxide, ruthenium nitride, an oxide containing strontium andruthenium, an oxide containing lanthanum and nickel, or the like ispreferably used. Tantalum nitride, titanium nitride, a nitridecontaining titanium and aluminum, a nitride containing tantalum andaluminum, ruthenium oxide, ruthenium nitride, an oxide containingstrontium and ruthenium, and an oxide containing lanthanum and nickelare preferable because they are oxidation-resistant conductive materialsor materials that retain their conductivity even after absorbing oxygen.Alternatively, a semiconductor having high electric conductivity,typified by polycrystalline silicon containing an impurity element suchas phosphorus, or silicide such as nickel silicide may be used.

Conductive layers formed using any of the above materials may bestacked. For example, a stacked-layer structure combining a materialcontaining any of the above metal elements and a conductive materialcontaining oxygen may be used. Alternatively, a stacked-layer structurecombining a material containing any of the above metal elements and aconductive material containing nitrogen may be used. Furtheralternatively, a stacked-layer structure combining a material containingany of the above metal elements, a conductive material containingoxygen, and a conductive material containing nitrogen may be used.

When an oxide is used for the channel formation region of thetransistor, the conductor functioning as the gate electrode preferablyemploys a stacked-layer structure using a material containing any of theabove metal elements and a conductive material containing oxygen. Inthis case, the conductive material containing oxygen is preferablyprovided on the channel formation region side. When the conductivematerial containing oxygen is provided on the channel formation regionside, oxygen released from the conductive material is easily supplied tothe channel formation region.

It is particularly preferable to use, for the conductor functioning asthe gate electrode, a conductive material containing oxygen and a metalelement contained in the metal oxide in which the channel is formed. Aconductive material containing any of the above metal elements andnitrogen may be used. For example, a conductive material containingnitrogen, such as titanium nitride or tantalum nitride, can be used.Indium tin oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon is added may be used. Indium gallium zincoxide containing nitrogen may be used. With the use of such a material,hydrogen contained in the metal oxide in which the channel is formed canbe captured in some cases. Hydrogen entering from a surroundinginsulator or the like can also be captured in some cases.

FIGS. 11A to 11D illustrate a semiconductor device including atransistor 200B. FIG. 11A is a top view of the semiconductor device, andFIGS. 11B, 11C, and 11D are cross-sectional views taken alongdashed-dotted lines A1-A2, A3-A4, and A5-A6 illustrated in FIG. 11A,respectively. FIG. 11B can also be referred to as a cross-sectional viewof the transistor 200B in the channel length direction. FIG. 11C canalso be referred to as a cross-sectional view of the transistor 200B inthe channel width direction. Note that for simplification, somecomponents are not illustrated in FIG. 11A.

The transistor 200B is different from the transistor 200A in that theshape of the insulator 283 is different from that in the transistor200A, insulators 287, 274, 271 a, and 271 b are included, and the oxides230 c and 230 d are not included.

In the semiconductor device illustrated in FIGS. 11A to 11D, theinsulators 214, 216, 222, 224, 254, 280, and 282 are patterned. Theinsulators 287 and 283 cover the insulators 214, 216, 222, 224, 254,280, and 282. That is, the insulator 287 is in contact with the topsurface of the insulator 212, the side surfaces of the insulators 214,216, 222, 224, 254, 280, and 282, and the top surface of the insulator282, and the insulator 283 is in contact with the top and side surfacesof the insulator 287. Accordingly, the oxide 230, the insulators 214,216, 222, 224, 254, 280, and 282, and the like are separated from theoutside by the insulators 287, 283, and 212. In other words, thetransistor 200B is placed in a region sealed by the insulators 287, 283,and 212.

The insulators 214, 282, and 287 are preferably formed using a materialhaving a function of capturing and fixing hydrogen, and the insulators212 and 283 are preferably formed using a material having a function ofinhibiting diffusion of hydrogen and oxygen. Typically, aluminum oxidecan be used for the insulators 214, 282, and 287, and silicon nitridecan be used for the insulators 212 and 283.

With the above structure, entry of hydrogen contained in the regionoutside the sealed region into the sealed region can be inhibited. Thus,the hydrogen concentration in the transistor can kept low.

Note that although the insulators 212, 287, and 283 are each a singlelayer in the transistor 200B illustrated in FIGS. 11A to 11D, theinsulators 212, 287, and 283 may each shave a single-layer structure ora stacked-layer structure.

The insulator 287 is not necessarily provided. When the insulator 287 isnot provided, the transistor 200B is provided in a region sealed by theinsulators 212 and 283. With the above structure, entry of hydrogencontained in the region outside the sealed region into the sealed regioncan be inhibited. Thus, the hydrogen concentration in the transistor canbe kept low.

The insulator 274 functions as an interlayer film. The insulator 274preferably has a lower permittivity than the insulator 214. The use of amaterial having a low dielectric constant for the interlayer film canreduce the parasitic capacitance between wirings. The insulator 274 canbe formed using a material similar to that for the insulator 280, forexample.

The insulating film to be the insulator 274 is preferably formed by asputtering method. A film formed by a sputtering method is preferablebecause of its low hydrogen concentration. Therefore, an increase in thehydrogen concentration in the transistor in the step of forming theinsulating film can be inhibited.

Note that it is preferable to form the insulators 287 and 283 and theabove-described insulating film successively without exposure to theair. Film formation without exposure to the air can prevent attachmentof impurities or moisture from the air onto the insulators 287 and 283,so that an interface between the insulators 287 and 283 and the vicinityof thereof and an interface between the insulator 283 and theabove-described insulating film and the vicinity of thereof can be keptclean. Moreover, the manufacturing process of the semiconductor devicecan be simplified.

In the semiconductor device illustrated in FIG. 11 , the insulator 271 ais provided between the conductor 242 a and the insulator 254, and theinsulator 271 b is provided between the conductor 242 b and theinsulator 254.

Here, the insulators 271 a and 271 b preferably have a function ofinhibiting oxygen diffusion. The structure enables less absorption ofexcess oxygen in the insulator 280 into the conductors 242 a and 242 bfunctioning as the source electrode and the drain electrode.Furthermore, by inhibiting oxidation of the conductors 242 a and 242 b,an increase in the contact resistance between the transistor 200B and awiring can be inhibited. Consequently, the transistor 200B can havefavorable electrical characteristics and reliability. The insulators 271a and 271 b can be formed using a material similar to that for formingthe insulator 254, for example.

In the manufacturing method of the semiconductor device illustrated inFIGS. 11A to 11D, an insulating film to be the insulators 271 a and 271b and a conductive layer provided over the insulating film can eachfunction as a mask when the conductor 242 is formed. Thus, an endportion at the intersection of the side surface and the top surface ofthe conductor 242 (the conductor 242 a and the conductor 242 b) isangular. The cross-sectional area of the conductor 242 is larger in thecase where an end portion at the intersection of the side surface andthe top surface of the conductor 242 is angular than in the case wherethe end portion is rounded. Accordingly, the resistance of the conductor242 is reduced, so that the on-state current of the transistor 200B canbe increased.

When the oxides 230 c and 230 d are not provided, generation of aparasitic transistor between two adjacent transistors 200B can beinhibited, which inhibits generation of a leakage path along theconductor 260. Thus, a semiconductor device that has favorableelectrical characteristics and can be miniaturized or highly integratedcan be provided.

This embodiment can be combined with the other embodiments asappropriate.

Embodiment 3

In this embodiment, a pixel of a display device of one embodiment of thepresent invention will be described with reference to FIG. 12 .

[Pixel]

A display device of this embodiment includes a plurality of pixelsarranged in a matrix of m rows and n columns (m and n are each aninteger of 1 or more). FIG. 12 shows an example of a circuit diagram ofa pixel 200(i,j) (i is an integer of 1 or more and m or less, and j isan integer of 1 or more and n or less).

The pixel 200(i,j) illustrated in FIG. 12 includes a light-emittingelement 210, a switch SW21, a switch SW22, a transistor M, and acapacitor C1.

In this embodiment, a transistor is used as the switch SW21. A gate ofthe switch SW21 is electrically connected to a scan line GL1(i). One ofa source and a drain of the switch SW21 is electrically connected to asignal line SL(j) and the other of the source and the drain iselectrically connected to a gate of the transistor M.

In this embodiment, a transistor is used as the switch SW22. A gate ofthe switch SW22 is electrically connected to a scan line GL2(i). One ofa source and a drain of the switch SW22 is electrically connected to awiring COM and the other of the source and the drain is electricallyconnected to the gate of the transistor M.

The gate of the transistor M is electrically connected to one electrodeof the capacitor C1, the other of the source and the drain of the switchSW21, and the other of the source and the drain of the switch SW22. Oneof a source and a drain of the transistor M is electrically connected toa wiring CATHODE and the other of the source and the drain iselectrically connected to a cathode of the light-emitting element 210.

The other electrode of the capacitor C1 is electrically connected to thewiring CATHODE.

An anode of the light-emitting element 210 is electrically connected toa wiring ANODE.

The scan line GL1(i) has a function of supplying a selection signal. Thescan line GL2(i) has a function of supplying a control signal. Thesignal line SL(j) has a function of supplying an image signal. Aconstant potential is supplied to each of the wiring COM, the wiringCATHODE, and the wiring ANODE. In the light-emitting element 210, theanode side can have a high potential and the cathode side can have alower potential than the anode side.

The switch SW21 is controlled by a selection signal and functions as aselection transistor for controlling the selection state of the pixel200(i,j).

The transistor M functions as a driving transistor that controls acurrent flowing through the light-emitting element 210 in accordancewith a potential supplied to the gate. When the switch SW21 is on, animage signal supplied to the signal line SL(j) is supplied to the gateof the transistor M, and the luminance of the light-emitting element 210can be controlled in accordance with the potential of the image signal.

The switch SW22 has a function of controlling the gate potential of thetransistor M on the basis of a control signal. Specifically, the switchSW22 can supply a potential for turning off the transistor M to the gateof the transistor M.

The switch SW22 can be used to control the pulse width, for example.During a period based on a control signal, a current can be suppliedfrom the transistor M to the light-emitting element 210. Thelight-emitting element 210 can express gray levels on the basis of animage signal and a control signal.

Here, as each of the transistors included in the pixel 200(i,j), it ispreferable to use a transistor including a metal oxide (an oxidesemiconductor) for a semiconductor layer where a channel is formed.

A transistor including a metal oxide which has a wider bandgap and alower carrier density than silicon has an extremely low off-statecurrent. Therefore, owing to the low off-state current, a chargeaccumulated in a capacitor that is series-connected to the transistorcan be retained for a long time. Thus, in particular, a transistorincluding an oxide semiconductor is preferably used as each of theswitches SW21 and SW22 series-connected to the capacitor C1. When eachof the other transistors also includes an oxide semiconductor,manufacturing cost can be reduced.

Alternatively, a transistor including silicon as a semiconductor inwhich a channel is formed can be used as the transistor included in thepixel 200(i,j). It is particularly preferable to use silicon with highcrystallinity such as single crystal silicon or polycrystalline siliconbecause high field-effect mobility can be achieved and higher-speedoperation can be performed.

Alternatively, a structure may be employed in which a transistorincluding an oxide semiconductor is used as one or more of thetransistors included in the pixel 200(i,j), and a transistor includingsilicon is used as the other transistor(s).

Note that although the transistor is an n-channel transistor in FIG. 12, a p-channel transistor can also be used.

There is no particular limitation on the structure of the transistor inthe display device. For example, a planar transistor, a staggeredtransistor, or an inverted staggered transistor may be used. A top-gatetransistor or a bottom-gate transistor may be used. Gate electrodes maybe provided above and below a channel.

As the transistor included in the display device, an OS transistor canbe used, for example. Therefore, a transistor with an extremely lowoff-state current can be provided.

Furthermore, a Si transistor may be used as the transistor included inthe display device. Examples of the transistor include a transistorcontaining amorphous silicon, a transistor containing crystallinesilicon (typically, low-temperature polysilicon), and a transistorcontaining single crystal silicon.

This embodiment can be combined with the other embodiments asappropriate.

Embodiment 4

Described in this embodiment is a metal oxide (also referred to as anoxide semiconductor) applicable to an OS transistor described in theabove embodiment.

The metal oxide preferably contains at least indium or zinc. Inparticular, indium and zinc are preferably contained. In addition,aluminum, gallium, yttrium, tin, or the like is preferably contained.Furthermore, one or more elements selected from boron, silicon,titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum,cerium, neodymium, hafnium, tantalum, tungsten, magnesium, cobalt, andthe like may be contained.

<Classification of Crystal Structure>

First, the classification of the crystal structures of an oxidesemiconductor will be explained with FIG. 13A. FIG. 13A is a diagramshowing the classification of crystal structures of an oxidesemiconductor, typically IGZO (a metal oxide containing In, Ga, and Zn).

The oxide semiconductor is classified into “amorphous”, “crystalline”,and “crystal”, as shown in FIG. 13A. “Amorphous” includes a completelyamorphous structure. “Crystalline” includes c-axis-aligned crystalline(CAAC), nanocrystalline (nc), and cloud-aligned composite (CAC)structures. Note that “crystalline” excludes single crystal, polycrystal, and completely amorphous structures. “Crystal” includes singlecrystal and poly crystal structures.

The structure shown in the thick frame in FIG. 13A is a new crystallinephase, which is an intermediate state between “amorphous” and “crystal”.That is, this structure is completely different from “amorphous” and“crystal”, which are energetically unstable.

A crystal structure of a film or a substrate can be analyzed with anX-ray diffraction (XRD) spectrum. Here, XRD spectra of a quartz glasssubstrate and an IGZO film having a crystal structure classified into“Crystalline” (also referred to as Crystalline IGZO), which are obtainedby grazing-incidence XRD (GIXD) measurement, are shown in FIGS. 13(B)and 13(C), respectively. Note that a GIXD method is also referred to asa thin film method or a Seemann-Bohlin method. Hereinafter, the XRDspectra shown in FIGS. 13B and 13C, which are obtained by the GIXDmeasurement, are simply referred to as XRD spectra. FIG. 13B shows theXRD spectrum of the quartz glass substrate and FIG. 13C shows the XRDspectrum of the crystalline IGZO film. Note that the crystalline IGZOfilm of FIG. 13C has an atomic ratio of approximately In:Ga:Zn=4:2:3.The crystalline IGZO film of FIG. 13C has a thickness of 500 nm.

The peak of the XRD spectrum of the quartz glass substrate has abilaterally symmetrical shape, as shown by the arrows in FIG. 13B. Onthe other hand, the peak of the XRD spectrum of the crystalline IGZOfilm has a bilaterally asymmetrical shape, as shown by arrows in FIG.13C. The bilaterally asymmetrical peak shows the existence of crystal inthe film or the substrate. In other words, the crystal structure of thefilm or the substrate cannot be regarded as “amorphous” unless it has abilaterally symmetrical peak in the XRD spectrum. Note that in FIG. 13C,an IGZO crystal phase is explicitly denoted at 2θ of 31° or in thevicinity thereof. The bilaterally asymmetrical peak of the XRD spectrumis probably attributed to a diffraction peak derived from such a crystalphase (a fine crystal).

Specifically, interference of an X-ray scattered by atoms contained inIGZO probably contributes to a peak at 2θ=34° or in the vicinitythereof. In addition, the fine crystal probably contributes to the peakat 2θ=31° or in the vicinity thereof. In the XRD spectrum of thecrystalline IGZO film shown in FIG. 13C, the peak at 2θ of 34° or in thevicinity thereof is wide on the lower angle side. This indicates thatthe crystalline IGZO film includes a fine crystal attributed to the peakat 2θ of 31° or in the vicinity thereof.

A crystal structure of a film or a substrate can be analyzed with adiffraction pattern obtained by nanobeam electron diffraction (NBED)(also referred to as a nanobeam electron diffraction pattern).Diffraction patterns of the quartz glass substrate and the IGZO filmformed with a substrate temperature set at room temperature are shown inFIGS. 13D and 13E, respectively. FIG. 13D shows the diffraction patternof the quartz glass substrate and FIG. 13E shows the diffraction patternof the IGZO film. Note that the IGZO film of FIG. 13E is formed by asputtering method using an In—Ga—Zn oxide target having an atomic ratioof In:Ga:Zn=1:1:1. The nanobeam electron diffraction is performed withan electron beam with a probe diameter of 1 nm.

Note that as shown in FIG. 13D, a halo pattern is observed in thediffraction pattern of the quartz glass substrate, which indicates thatthe quartz glass substrate is in an amorphous state. As shown in FIG.13E, not a halo pattern but a spot-like pattern is observed in thediffraction pattern of the IGZO film formed at room temperature. Thus,it is presumed that the IGZO film formed at room temperature is in anintermediate state, which is neither a crystal state nor an amorphousstate, and it cannot be concluded that the IGZO film is in an amorphousstate.

<<Oxide Semiconductor Structure>>

Oxide semiconductors might be classified in a manner different from theabove-described one shown in FIG. 13A when classified in terms of thecrystal structure. Oxide semiconductors are classified into a singlecrystal oxide semiconductor and a non-single-crystal oxidesemiconductor, for example. Examples of the non-single-crystal oxidesemiconductor include the above-described CAAC-OS and nc-OS. Apolycrystalline oxide semiconductor, an amorphous-like oxidesemiconductor (a-like OS), and an amorphous oxide semiconductor are alsogiven as examples of the non-single-crystal oxide semiconductors.

Here, the above-described CAAC-OS, nc-OS, and a-like OS will bedescribed in detail.

[CAAC-OS]

The CAAC-OS is an oxide semiconductor that has a plurality of crystalregions each of which has c-axis alignment in a particular direction.Note that the particular direction refers to the film thicknessdirection of a CAAC-OS film, the normal direction of the surface wherethe CAAC-OS film is formed, or the normal direction of the surface ofthe CAAC-OS film. The crystal region refers to a region having aperiodic atomic arrangement. When an atomic arrangement is regarded as alattice arrangement, the crystal region also refers to a region with auniform lattice arrangement. The CAAC-OS has a region where a pluralityof crystal regions are connected in the a-b plane direction, and theregion has distortion in some cases. Note that distortion refers to aportion where the direction of a lattice arrangement changes between aregion with a uniform lattice arrangement and another region with auniform lattice arrangement in a region where a plurality of crystalregions are connected. That is, the CAAC-OS is an oxide semiconductorhaving c-axis alignment and having no clear alignment in the a-b planedirection.

Note that each of the plurality of crystal regions is formed of one ormore fine crystals (crystals each of which has a maximum diameter ofless than 10 nm). In the case where the crystal region is formed of onefine crystal, the maximum diameter of the crystal region is less than 10nm. In the case where the crystal region is formed of a large number offine crystals, the size of the crystal region may be approximatelyseveral tens of nanometers.

Furthermore, in an In-M-Zn oxide (the element M is one or more ofaluminum, gallium, yttrium, tin, titanium, and the like), the CAAC-OStends to have a layered crystal structure (also referred to as a layeredstructure) in which a layer containing indium (In) and oxygen(hereinafter, In layer) and a layer containing the element M, zinc (Zn),and oxygen (hereinafter, (M,Zn) layer) are stacked. Indium and theelement M can be replaced with each other. Therefore, indium may becontained in the (M,Zn) layer. In addition, the element M may becontained in the In layer. Note that Zn may be contained in the Inlayer. Such a layered structure is observed as a lattice image in ahigh-resolution TEM image, for example.

When the CAAC-OS film is subjected to structural analysis byout-of-plane XRD measurement with an XRD apparatus using a θ/2θ scan,for example, a peak indicating c-axis alignment is detected at 2θ=31° orin the vicinity thereof. Note that the position of the peak indicatingc-axis alignment (the value of 2θ) may change depending on the kind,composition, or the like of the metal element contained in the CAAC-OS.

Moreover, in the electron diffraction pattern of the CAAC-OS film, aplurality of bright spots appear, for example. Note that one spot andanother spot are symmetric with respect to a spot of the incidentelectron beam which passes through a sample (also referred to as adirect spot).

In the case where the crystal region is observed from theabove-described specific direction, the shape of the lattice alignmentin the crystal region is basically a hexagon but is not always a regularhexagon and is a non-regular hexagon in some cases. A pentagonal latticearrangement, a heptagonal lattice arrangement, and the like are includedin the distortion in some cases. Note that a clear grain boundary cannotbe observed even in the vicinity of distortion in the CAAC-OS. That is,formation of a grain boundary is inhibited by the distortion of alattice arrangement. This is probably because the CAAC-OS can toleratedistortion owing to a low density of arrangement of oxygen atoms in thea-b plane direction, an interatomic bond distance changed bysubstitution of a metal atom, and the like.

A crystal structure in which a clear grain boundary is observed is aso-called poly crystal structure. It is highly possible that the grainboundary serves as a recombination center and traps a carrier, and thusthe on-state current and field-effect mobility of a transistor aredecreased, for example. Thus, the CAAC-OS in which no clear grainboundary is observed is one of oxides with crystallinity having afavorable crystal structure for a semiconductor layer of a transistor.Note that the CAAC-OS preferably contains Zn. For example, an In—Znoxide and an In—Ga—Zn oxide are suitable because generation of grainboundaries can be better prevented than in an In oxide.

The CAAC-OS is an oxide semiconductor with high crystallinity in whichno clear grain boundary is observed. Thus, in the CAAC-OS, a reductionin electron mobility due to the grain boundary is less likely to occur.Entry of impurities, formation of defects, or the like might decreasethe crystallinity of an oxide semiconductor. This means that the CAAC-OScan be referred to as an oxide semiconductor having small amounts ofimpurities and defects (e.g., oxygen vacancies). Thus, an oxidesemiconductor including the CAAC-OS is physically stable. Accordingly,the oxide semiconductor including the CAAC-OS is resistant to heat andhas high reliability. In addition, the CAAC-OS is stable with respect tohigh temperature in the manufacturing process (what is called thermalbudget). Accordingly, the use of the CAAC-OS for the OS transistor canextend a degree of freedom of the manufacturing process.

[Nc-OS]

In the nc-OS, a microscopic region (e.g., a region with a size greaterthan or equal to 1 nm and less than or equal to 10 nm, in particular, aregion with a size greater than or equal to 1 nm and less than or equalto 3 nm) has a periodic atomic arrangement. In other words, the nc-OSincludes a fine crystal. Note that the size of the fine crystal isgreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, greater than or equal to 1 nm and less than or equal to 3nm, and thus the fine crystal is also referred to as a nanocrystal.There is no regularity of crystal orientation between differentnanocrystals in the nc-OS. Thus, the orientation in the whole film isnot observed. Accordingly, in some cases, the nc-OS cannot bedistinguished from an a-like OS or an amorphous oxide semiconductor,depending on an analysis method. For example, when the nc-OS film issubjected to structural analysis by out-of-plane XRD measurement with anXRD apparatus using a θ/2θ scan, no peak indicating crystallinity isdetected. Furthermore, a halo pattern is found in a selected-areaelectron diffraction pattern of the nc-OS film obtained by using anelectron beam having a probe diameter larger than the diameter of ananocrystal (e.g., larger than or equal to 50 nm). Meanwhile, in somecases, a plurality of spots are observed in a ring-like region whosecenter is a direct spot in an electron diffraction pattern (alsoreferred to as nanobeam electron diffraction pattern) of the nc-OS film,which is obtained using an electron beam with a probe diameter close toor smaller than the diameter of a nanocrystal (e.g., larger than orequal to 1 nm and smaller than or equal to 30 nm).

[A-Like OS]

The a-like OS is an oxide semiconductor having a structure between thoseof the nc-OS and the amorphous oxide semiconductor. The a-like OS has avoid or a low-density region. That is, the a-like OS has lowercrystallinity than the nc-OS and the CAAC-OS. Moreover, the a-like OShas higher hydrogen concentration than the nc-OS and the CAAC-OS.

<<Oxide Semiconductor Structure>>

Next, the above-described CAC-OS will be described in detail. Note thatthe CAC-OS relates to the material composition.

[CAC-OS]

The CAC-OS has, for example, a composition in which elements included ina metal oxide are unevenly distributed. Materials including unevenlydistributed elements each have a size greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 3 nm, or a similar size. Note that in thefollowing description of a metal oxide, a state in which one or moremetal elements are unevenly distributed and regions including the metalelement(s) are mixed is referred to as a mosaic pattern or a patch-likepattern. The regions each have a size greater than or equal to 0.5 nmand less than or equal to 10 nm, preferably greater than or equal to 1nm and less than or equal to 3 nm, or a similar size.

The CAC-OS has a composition in which materials are separated into afirst region and a second region, and a mosaic pattern is formed. Thefirst regions are distributed in the film. Hereinafter, this compositionis also referred to as a cloud-like composition. In other words, theCAC-OS is a composite metal oxide having a composition in which thefirst regions and the second regions are mixed.

Here, the atomic ratios of In, Ga, and Zn to a metal element included ina CAC-OS in an In—Ga—Zn oxide are expressed as [In], [Ga], and [Zn],respectively. For example, the first region of the CAC-OS in theIn—Ga—Zn oxide has [In], which is greater than that in the compositionof a CAC-OS film. In addition, the second region of the CAC-OS in theIn—Ga—Zn oxide has [Ga], which is greater than that in the compositionof the CAC-OS film. Alternatively, for example, [In] of the first regionis greater than that in the second region, and [Ga] of the first regionis less than that in the second region. In addition, [Ga] of the secondregion is greater than that in the first region, and [In] of the secondregion is less than that in the first region.

Specifically, the first region includes indium oxide, indium zinc oxide,or the like as a main component. The second region includes galliumoxide, gallium zinc oxide, or the like as a main component. That is, thefirst region can also be referred to as a region containing In as a maincomponent. The second region can also be referred to as a regioncontaining Ga as a main component.

Note that a boundary between the first region and the second region isnot clearly observed in some cases.

In a material composition of a CAC-OS in an In—Ga—Zn oxide that containsIn, Ga, Zn, and O, regions containing Ga as a main component areobserved in part of the CAC-OS and regions containing In as a maincomponent are observed in part thereof. These regions are randomlydispersed to form a mosaic pattern. Thus, it is suggested that theCAC-OS has a structure in which metal elements are unevenly distributed.

The CAC-OS can be formed by a sputtering method under a condition wherea substrate is intentionally not heated, for example. In the case offorming the CAC-OS by a sputtering method, one or more selected from aninert gas (typically, argon), an oxygen gas, and a nitrogen gas may beused as a deposition gas. The ratio of the flow rate of an oxygen gas tothe total flow rate of the deposition gas at the time of film formationis preferably as low as possible, and for example, the flow ratio of anoxygen gas is preferably higher than or equal to 0% and less than 30%,further preferably higher than or equal to 0% and less than or equal to10%.

For example, energy dispersive X-ray spectroscopy (EDX) is used toobtain EDX mapping, and according to the EDX mapping, the CAC-OS in theIn—Ga—Zn oxide has a composition in which the regions containing In as amain component (the first regions) and the regions containing Ga as amain component (the second regions) are unevenly distributed and mixed.

Here, the first region has a higher conductivity than the second region.In other words, when carriers flow through the first region, theconductivity of a metal oxide is exhibited. Accordingly, when the firstregions are distributed in a metal oxide as a cloud, high field-effectmobility (μ) can be achieved.

The second region has a higher insulating property than the firstregion. In other words, when the second regions are distributed in ametal oxide, leakage current can be inhibited.

Thus, in the case where a CAC-OS is used for a transistor, by thecomplementary function of the conducting function due to the firstregion and the insulating function due to the second region, the CAC-OScan have a switching function (on/off function). That is, a CAC-OS has aconducting function in part of the material and has an insulatingfunction in another part of the material; as a whole, the CAC-OS has afunction of a semiconductor. Separation of the conducting function andthe insulating function can maximize each function. Accordingly, when aCAC-OS is used for a transistor, a high on-state current (Ion), a highfield-effect mobility (4 and favorable switching operation can beachieved.

A transistor including a CAC-OS is highly reliable. Thus, the CAC-OS issuitably used in a variety of semiconductor devices typified by adisplay device.

An oxide semiconductor can have any of various structures that showvarious different properties. Two or more of the amorphous oxidesemiconductor, the polycrystalline oxide semiconductor, the a-like OS,the CAC-OS, the nc-OS, and the CAAC-OS may be included in an oxidesemiconductor of one embodiment of the present invention.

<Transistor Including Oxide Semiconductor>

Next, a transistor including the above oxide semiconductor is described.

When the oxide semiconductor is used for a transistor, the transistorcan have high field-effect mobility. In addition, the transistor canhave high reliability.

The transistor preferably has an oxide semiconductor with a low carrierconcentration. For example, the carrier density of a metal oxide islower than or equal to 1×10¹⁷ cm⁻³, preferably lower than or equal to1×10¹⁵ cm⁻³, further preferably lower than or equal to 1×10¹³ cm⁻³,still further preferably lower than or equal to 1×10¹¹ cm⁻³, yet furtherpreferably lower than 1×10¹⁰ cm⁻³, and higher than or equal to 1×10⁻⁹cm⁻³. In order to reduce the carrier density of an oxide semiconductorfilm, the impurity concentration in the oxide semiconductor film isreduced so that the density of defect states can be reduced. In thisspecification and the like, a state with a low concentration ofimpurities and a low density of defect states is referred to as a highlypurified intrinsic or substantially highly purified intrinsic state.Note that an oxide semiconductor with a low carrier concentration may bereferred to as a highly purified intrinsic or substantially highlypurified intrinsic oxide semiconductor.

A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film has a low density of defect states andaccordingly has a low density of trap states in some cases.

Charges trapped by the trap states in an oxide semiconductor take a longtime to be released and may behave like fixed charges. A transistorwhose channel formation region is formed in an oxide semiconductorhaving a high density of trap states has unstable electricalcharacteristics in some cases.

In order to obtain stable electrical characteristics of the transistor,it is effective to reduce impurity concentrations in the oxidesemiconductor. In order to reduce the concentration of impurities in theoxide semiconductor, impurity concentrations in a film that is adjacentto the oxide semiconductor is preferably reduced. Examples of impuritiesinclude hydrogen, nitrogen, alkali metal, alkaline earth metal, iron,nickel, and silicon.

<Impurities>

The influence of impurities in the oxide semiconductor is described.

When silicon or carbon, which is a Group 14 element, is contained in anoxide semiconductor, defect states are formed in the oxidesemiconductor. Thus, the concentration of silicon or carbon in the oxidesemiconductor and in the vicinity of an interface with the oxidesemiconductor (measured by secondary ion mass spectrometry (SIMS)) islower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁷ atoms/cm³.

When the oxide semiconductor contains alkali metal or alkaline earthmetal, defect states are formed and carriers are generated, in somecases. Accordingly, a transistor including an oxide semiconductor thatcontains alkali metal or alkaline earth metal tends to have normally-oncharacteristics. Thus, the concentration of alkali metal or alkalineearth metal in the oxide semiconductor measured by SIMS is set lowerthan or equal to 1×10¹⁸ atoms/cm³, and preferably lower than or equal to2×10¹⁶ atoms/cm³.

An oxide semiconductor containing nitrogen easily becomes n-type bygeneration of electrons serving as carriers and an increase in carrierconcentration. A transistor including an oxide semiconductor thatcontains nitrogen as the semiconductor tends to have normally-oncharacteristics. When nitrogen is contained in the oxide semiconductor,a trap state is sometimes formed. This might make the electricalcharacteristics of the transistor unstable. Thus, the concentration ofnitrogen in the oxide semiconductor, which is measured by SIMS, can belower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³,still more preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide semiconductor reacts with oxygen, whichreacts with metal atoms, to be water, and thus causes an oxygen vacancyin some cases. Entry of hydrogen into the oxygen vacancy generates anelectron serving as a carrier in some cases. Furthermore, some hydrogenmay react with oxygen bonded to a metal atom and generate an electronserving as a carrier. Thus, a transistor including an oxidesemiconductor that contains hydrogen tends to have normally-oncharacteristics. Accordingly, hydrogen in the oxide semiconductor ispreferably reduced as much as possible. Specifically, the hydrogenconcentration in the oxide semiconductor measured by SIMS is lower than1×10²⁰ atoms/cm³, preferably lower than 1×10¹⁹ atoms/cm³, furtherpreferably lower than 5×10¹⁸ atoms/cm³, still further preferably lowerthan 1×10¹⁸ atoms/cm³.

An oxide semiconductor with sufficiently reduced impurities is used fora channel formation region of a transistor, so that the transistor canhave stable electrical characteristics.

This embodiment can be combined with the other embodiments asappropriate.

Embodiment 5

In this embodiment, electronic devices of embodiments of the presentinvention will be described with reference to FIGS. 14A and 14B, FIGS.15A and 15B, FIGS. 16A and 16B, FIGS. 17A to 17D, and FIGS. 18A to 18F.

Electronic devices of this embodiment are each provided with a displaydevice of one embodiment of the present invention in a display portion.A display device of one embodiment of the present invention has highdisplay quality and low power consumption. In addition, the displaydevice of one embodiment of the present invention can be easilyincreased in resolution and size. Thus, the display device of oneembodiment of the present invention can be used for a display portion ofa variety of electronic devices.

Examples of the electronic devices include a digital camera, a digitalvideo camera, a digital photo frame, a mobile phone, a portable gameconsole, a portable information terminal, and an audio reproducingdevice, in addition to electronic devices with a relatively largescreen, such as a television device, a desktop or laptop personalcomputer, a monitor of a computer or the like, digital signage, and alarge game machine such as a pachinko machine.

In particular, a display device of one embodiment of the presentinvention can have a high resolution, and thus can be favorably used foran electronic device having a relatively small display portion. Thedisplay device can be favorably used for, for example, an electronicdevice include a watch-type or bracelet-type information terminal device(wearable device) and a wearable device worn on a head, such as a VRdevice such as a head mounted display, a glasses-type AR device, and anMR device.

The electronic device in this embodiment may include a sensor (a sensorhaving a function of measuring force, displacement, position, speed,acceleration, angular velocity, rotational frequency, distance, light,liquid, magnetism, temperature, a chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, gradient, oscillation, a smell, or infrared rays).

The electronic device in this embodiment can have a variety offunctions. For example, the electronic device of one embodiment of thepresent invention can have a function of displaying a variety of data (astill image, a moving image, a text image, and the like) on the displayportion, a touch panel function, a function of displaying a calendar,date, time, and the like, a function of executing a variety of software(programs), a wireless communication function, and a function of readingout a program or data stored in a recording medium.

FIG. 14A is a perspective view of a glasses-type electronic device 900.The electronic device 900 includes a pair of display panels 901, a pairof housings 902, a pair of optical members 903, a pair of temples 904,and the like.

The electronic device 900 can project an image displayed on the displaypanel 901 onto a display region 906 of the optical member 903. Since theoptical members 903 have a light-transmitting property, a user can seeimages displayed on the display regions 906, which are superimposed ontransmission images seen through the optical members 903. Thus, theelectronic device 900 is an electronic device capable of AR display.

The display panel 901 included in the electronic device 900 preferablyhas a function of taking an image in addition to a function ofdisplaying an image. In that case, the electronic device 900 can receivelight incident on the display panel 901 through the optical member 903,convert the light into an electric signal, and output the electricsignal. Therefore, an image of the user's eye or the user's eye and thevicinity thereof can be taken and the image can be output to the outsideor an arithmetic portion included in the electronic device 900 as imageinformation.

The housings 902 each include a camera 905 capable of taking an image ofwhat lies in front thereof. Although not illustrated, one of thehousings 902 is provided with a wireless receiver or a connector towhich a cable can be connected, whereby a video signal or the like canbe supplied to the housing 902. Furthermore, when the housing 902 isprovided with an acceleration sensor such as a gyroscope sensor, theorientation of the user's head can be detected and an imagecorresponding to the orientation can be displayed on the display region906. Moreover, the housing 902 is preferably provided with a batterycapable of being charged with or without a wire.

Next, a method for projecting an image on the display region 906 of theelectronic device 900 is described with reference to FIG. 14B. Thedisplay panel 901, a lens 911, and a reflective plate 912 are providedin the housing 902. A reflective surface 913 functioning as a halfmirror is provided as a portion corresponding to the display region 906of the optical member 903.

Light 915 emitted from the display panel 901 passes through the lens 911and is reflected by the reflective plate 912 toward the optical member903. In the optical member 903, the light 915 is fully reflectedrepeatedly by surfaces of an edge portion of the optical member 903 andreaches the reflective surface 913, whereby an image is projected on thereflective surface 913. Accordingly, the user can see both the light 915reflected by the reflective surface 913 and transmitted light 916 thatpasses through the optical member 903 (including the reflective surface913).

FIGS. 14A and 14B show an example in which the reflective plate 912 andthe reflective surface 913 each have a curved surface. This structurecan increase optical design flexibility and reduce the thickness of theoptical member 903, compared to the case where the reflective plate 912and the reflective surface 913 are flat. Note that the reflective plate912 and the reflective surface 913 may be flat.

The reflective plate 912 can be a component having a mirror surface andpreferably has high reflectivity. As the reflective surface 913, a halfmirror utilizing reflection of a metal film may be used, but the use ofa total-reflection prism or the like can increase the transmittance ofthe transmitted light 916.

Here, the electronic device 900 preferably includes a mechanism foradjusting the distance and angle between the lens 911 and the displaypanel 901, in which case the focus can be adjusted and images can bezoomed in and out, for example. At least one of the lens 911 and thedisplay panel 901 is preferably configured to be movable in theoptical-axis direction, for example.

The electronic device 900 preferably includes a mechanism capable ofadjusting the angle of the reflective plate 912. The position of thedisplay region 906 where images are displayed can be changed by changingthe angle of the reflective plate 912. Thus, the display region 906 canbe placed at the most appropriate position in accordance with theposition of the user's eye.

The display device of one embodiment of the present invention can beused for the display panel 901. Thus, the electronic device 900 capableof extremely-high-resolution display can be provided.

FIGS. 15A and 15B are perspective views of a goggle-type electronicdevice 950. FIG. 15A is a perspective view illustrating the frontsurface, the top surface, and the left side surface of the electronicdevice 950, and FIG. 15B is a perspective view illustrating the backsurface, the bottom surface, and the right side surface of theelectronic device 950.

The electronic device 950 includes a pair of display panels 951, ahousing 952, a pair of temples 954, a cushion 955, a pair of lenses 956,and the like. The pair of display panels 951 is positioned to be seenthrough the lenses 956 inside the housing 952.

The electronic device 950 is a VR electronic device. A user wearing theelectronic device 950 can see an image displayed on the display panel951 through the lens 956. Furthermore, the pair of display panels 951may display different images, whereby three-dimensional display usingparallax can be performed.

An input terminal 957 and an output terminal 958 are provided on theback side of the housing 952. To the input terminal 957, a cable forsupplying a video signal from a video output device or the like, powerfor charging a battery provided in the housing 952, or the like can beconnected. The output terminal 958 can function as, for example, anaudio output terminal to which earphones, headphones, or the like can beconnected. Note that in the case where audio data can be output bywireless communication or sound is output from an external video outputdevice, the audio output terminal is not necessarily provided.

The electronic device 950 preferably includes a mechanism by which theleft and right positions of the lens 956 and the display panel 951 canbe adjusted to the optimal positions in accordance with the position ofthe user's eye. In addition, the electronic device 950 preferablyincludes a mechanism for adjusting the focal point by changing thedistance between the lens 956 and the display panel 951.

The display device of one embodiment of the present invention can beused for the display panel 951. Thus, the electronic device 950 capableof extremely-high-resolution display can be provided. As a result, auser can obtain a high sense of immersion.

The cushion 955 is in contact with the user's face (forehead, cheek, orthe like). The cushion 955 is in close contact with the user's face, sothat light leakage can be prevented, which increases the sense ofimmersion. It is preferable that a soft material be used for the cushion955 so that the cushion 955 is in close contact with the face of theuser wearing the electronic device 950. For example, a material such assilicone rubber, urethane, or sponge can be used. Furthermore, when asponge or the like whose surface is covered with cloth, leather (naturalleather or synthetic leather), or the like is used as the cushion 955, agap is less likely to be generated between the user's face and thecushion 955, whereby light leakage can be suitably prevented. The memberin contact with user's skin, such as the cushion 955 or the temple 954,is preferably detachable, in which case cleaning or replacement can beeasily performed.

An electronic device 6500 illustrated in FIG. 16A is a portableinformation terminal that can be used as a smartphone.

The electronic device 6500 includes a housing 6501, a display portion6502, a power button 6503, buttons 6504, a speaker 6505, a microphone6506, a camera 6507, a light source 6508, and the like. The displayportion 6502 has a touch panel function.

The display portion 6502 can use a display device of one embodiment ofthe present invention.

FIG. 16B is a schematic cross-sectional view including an end portion ofthe housing 6501 on the microphone 6506 side.

A protection member 6510 having a light-transmitting property isprovided on a display surface side of the housing 6501, and a displaypanel 6511, an optical member 6512, a touch sensor panel 6513, a printedcircuit board 6517, a battery 6518, and the like are provided in a spacesurrounded by the housing 6501 and the protection member 6510.

The display panel 6511, the optical member 6512, and the touch sensorpanel 6513 are fixed to the protection member 6510 with an adhesivelayer (not shown).

Part of the display panel 6511 is folded back in a region outside thedisplay portion 6502, and an FPC 6515 is connected to the part that isfolded back. An IC 6516 is mounted on the FPC 6515. The FPC 6515 isconnected to a terminal provided on the printed circuit board 6517.

A flexible display of one embodiment of the present invention can beused as the display panel 6511. Thus, an extremely lightweightelectronic device can be provided. Since the display panel 6511 isextremely thin, the battery 6518 with high capacity can be mounted withthe thickness of the electronic device controlled. An electronic devicewith a narrow frame can be obtained when part of the display panel 6511is folded back so that the portion connected to the FPC 6515 ispositioned on the rear side of a pixel portion.

FIG. 17A shows an example of a television device. In a television device7100, a display portion 7000 is incorporated in a housing 7101. Here, astructure in which the housing 7101 is supported by a stand 7103 isillustrated.

The display portion 7000 can use a display device of one embodiment ofthe present invention.

Operation of the television device 7100 illustrated in FIG. 17A can beperformed with an operation switch provided in the housing 7101 or aseparate remote controller 7111. Alternatively, the display portion 7000may include a touch sensor, and the television device 7100 may beoperated by touch on the display portion 7000 with a finger or the like.The remote controller 7111 may be provided with a display portion fordisplaying data output from the remote controller 7111. With operationkeys or a touch panel provided in the remote controller 7111, channelsand volume can be operated and videos displayed on the display portion7000 can be operated.

Note that the television device 7100 has a structure in which areceiver, a modem, and the like are provided. A general televisionbroadcast can be received with the receiver. When the television deviceis connected to a communication network with or without wires via themodem, one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver or between receivers, for example) datacommunication can be performed.

FIG. 17B shows an example of a laptop personal computer. A laptoppersonal computer 7200 includes a housing 7211, a keyboard 7212, apointing device 7213, an external connection port 7214, and the like. Inthe housing 7211, the display portion 7000 is incorporated.

The display portion 7000 can use a display device of one embodiment ofthe present invention.

FIGS. 17C and 17D show examples of digital signage.

Digital signage 7300 illustrated in FIG. 17C includes a housing 7301,the display portion 7000, a speaker 7303, and the like. Furthermore, thedigital signage can include an LED lamp, operation keys (including apower switch or an operation switch), a connection terminal, a varietyof sensors, a microphone, and the like.

FIG. 17D is digital signage 7400 attached to a cylindrical pillar 7401.The digital signage 7400 includes the display portion 7000 providedalong a curved surface of the pillar 7401.

In FIGS. 17C and 17D, the display portion 7000 can use a display deviceof one embodiment of the present invention.

A larger area of the display portion 7000 can increase the amount ofdata that can be provided at a time. The larger display portion 7000attracts more attention, so that the effectiveness of the advertisementcan be increased, for example.

The use of a touch panel in the display portion 7000 is preferablebecause in addition to display of a still image or a moving image on thedisplay portion 7000, intuitive operation by a user is possible.Moreover, for an application for providing information such as routeinformation or traffic information, usability can be enhanced byintuitive operation.

As illustrated in FIGS. 17C and 17D, it is preferable that the digitalsignage 7300 or the digital signage 7400 can work with an informationterminal 7311 or an information terminal 7411 such as a smartphone thata user has through wireless communication. For example, information ofan advertisement displayed on the display portion 7000 can be displayedon a screen of the information terminal 7311 or the information terminal7411. By operation of the information terminal 7311 or the informationterminal 7411, display on the display portion 7000 can be switched.

It is possible to make the digital signage 7300 or the digital signage7400 execute a game with the use of the screen of the informationterminal 7311 or the information terminal 7411 as an operation means(controller). Thus, an unspecified number of users can join in and enjoythe game concurrently.

Electronic devices shown in FIGS. 18A to 18F include a housing 9000, adisplay portion 9001, a speaker 9003, an operation key 9005 (including apower switch or an operation switch), a connection terminal 9006, asensor 9007 (a sensor having a function of measuring force,displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature, achemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, a smell, or infrared rays), a microphone 9008, and thelike.

The electronic devices shown in FIGS. 18A to 18F have a variety offunctions. For example, the electronic devices can have a function ofdisplaying a variety of data (a still image, a moving image, a textimage, and the like) on the display portion, a touch panel function, afunction of displaying a calendar, date, time, and the like, a functionof controlling processing with the use of a variety of software(programs), a wireless communication function, and a function of readingout and processing a program or data stored in a recording medium. Notethat the functions of the electronic devices are not limited thereto,and the electronic devices can have a variety of functions. Theelectronic devices may include a plurality of display portions. Theelectronic devices may each include a camera or the like and have afunction of taking a still image or a moving image and storing the takenimage in a recording medium (an external recording medium or a recordingmedium incorporated in the camera), a function of displaying the takenimage on the display portion, or the like.

The details of the electronic devices shown in FIGS. 18A to 18F will bedescribed below.

FIG. 18A is a perspective view showing a portable information terminal9101. For example, the portable information terminal 9101 can be used asa smartphone. Note that the portable information terminal 9101 may beprovided with the speaker 9003, the connection terminal 9006, the sensor9007, or the like. The portable information terminal 9101 can displaycharacters and image information on its plurality of surfaces. FIG. 18Ashows an example in which three icons 9050 are displayed. Information9051 indicated by dashed rectangles can be displayed on another surfaceof the display portion 9001. Examples of the information 9051 includenotification of reception of an e-mail, an SNS message, or an incomingcall, the title and sender of an e-mail, an SNS message, or the like,the date, the time, remaining battery, and the reception strength of anantenna. Alternatively, the icon 9050 or the like may be displayed inthe position where the information 9051 is displayed.

FIG. 18B is a perspective view showing a portable information terminal9102. The portable information terminal 9102 has a function ofdisplaying information on three or more surfaces of the display portion9001. Here, an example in which information 9052, information 9053, andinformation 9054 are displayed on different surfaces is shown. Forexample, the user can check the information 9053 displayed in a positionthat can be observed from above the portable information terminal 9102,with the portable information terminal 9102 put in a breast pocket ofhis/her clothes. The user can see the display without taking out theportable information terminal 9102 from the pocket and decide whether toanswer the call, for example.

FIG. 18C is a perspective view showing a watch-type portable informationterminal 9200. For example, the portable information terminal 9200 canbe used as a smart watch. The display surface of the display portion9001 is curved and provided, and display can be performed along thecurved display surface. Mutual communication between the portableinformation terminal 9200 and, for example, a headset capable ofwireless communication enables hands-free calling. With the connectionterminal 9006, the portable information terminal 9200 can perform mutualdata transmission with another information terminal and charging. Notethat the charging operation may be performed by wireless power feeding.

FIGS. 18D, 18E, and 18F are perspective views showing a foldableportable information terminal 9201. FIG. 18D is a perspective view of anopened state of the portable information terminal 9201, FIG. 18F is aperspective view of a folded state thereof, and FIG. 18E is aperspective view of a state in the middle of change from one of FIG. 18Dand FIG. 18F to the other. The portable information terminal 9201 ishighly portable in the folded state and is highly browsable in theopened state because of a seamless large display region. The displayportion 9001 of the portable information terminal 9201 is supported bythree housings 9000 joined by hinges 9055. For example, the displayportion 9001 can be folded with a radius of curvature of greater than orequal to 0.1 mm and less than or equal to 150 mm.

This embodiment can be combined with the other embodiments asappropriate.

This application is based on Japanese Patent Application Serial No.2019-115298 filed with Japan Patent Office on Jun. 21, 2019, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a first insulatinglayer; a second insulating layer; a first transistor; a secondtransistor; a first light-emitting diode; a second light-emitting diode;a coloring layer; and a color conversion layer, wherein the firsttransistor is electrically connected to the first light-emitting diode,wherein the second transistor is electrically connected to the secondlight-emitting diode, wherein the first insulating layer is over thefirst transistor and the second transistor, wherein the firstlight-emitting diode and the second light-emitting diode are over thefirst insulating layer, wherein the color conversion layer is over thesecond light-emitting diode, wherein the color conversion layer isconfigured to convert light emitted from the second light-emitting diodeinto a light having a longer wavelength, wherein the first transistorand the second transistor each comprise a metal oxide layer and a gateelectrode, wherein the metal oxide layer of the first transistorcomprises a channel formation region of the first transistor, whereinthe metal oxide layer of the second transistor comprises a channelformation region of the second transistor, wherein a top surface of thegate electrode of each of the first transistor and the second transistoris level or substantially level with a top surface of the secondinsulating layer due to a planarization treatment, and wherein thecoloring layer is provided over and in contact with a top surface andside surfaces of the color conversion layer.
 2. The display deviceaccording to claim 1, wherein the first transistor further comprises agate insulating layer, a first conductive layer, and a second conductivelayer, wherein the metal oxide layer of the first transistor comprises afirst region overlapping with the first conductive layer, a secondregion overlapping with the second conductive layer, and a third regionbetween the first region and the second region, wherein the firstconductive layer and the second conductive layer are apart from eachother over the metal oxide layer of the first transistor, wherein thesecond insulating layer is over the first conductive layer and thesecond conductive layer, wherein the second insulating layer comprisesan opening overlapping with the third region, wherein the gateinsulating layer is inside the opening and overlaps with a side surfaceof the second insulating layer and a top surface of the third region,and wherein the gate electrode of the first transistor is inside theopening and overlaps with the side surface of the second insulatinglayer and the top surface of the third region with the gate insulatinglayer therebetween.
 3. The display device according to claim 1, whereinthe color conversion layer is in contact with the second light-emittingdiode, and wherein the metal oxide layer of the first transistoroverlaps with the first light-emitting diode.
 4. The display deviceaccording to claim 1, further comprising a third insulating layer,wherein the third insulating layer is between the second light-emittingdiode and the color conversion layer, wherein the color conversion layeris in contact with the third insulating layer, and wherein no spacer isprovided between the first light-emitting diode and the secondlight-emitting diode.
 5. The display device according to claim 1,wherein each of the first light-emitting diode and the secondlight-emitting diode is a light-emitting diode comprising alight-emitting region whose area is less than or equal to 10000 μm²,wherein a third insulating layer is in contact with a first electrode ofthe first light-emitting diode and with a second electrode of the firstlight-emitting diode, and wherein the third insulating layer is incontact with a first electrode of the second light-emitting diode. 6.The display device according to claim 1, wherein the firstlight-emitting diode and the second light-emitting diode each emit bluelight, wherein a first conductive layer and a second conductive layerare provided on a third insulating layer provided over the secondinsulating layer and below the first light-emitting diode, and whereinthe first conductive layer and the second conductive layer overlap withthe first light-emitting diode.
 7. The display device according to claim1, wherein the first transistor and the second transistor are differentfrom each other in one or both of a channel length and a channel width.8. The display device according to claim 1, further comprising a drivercircuit and a fourth insulating layer, wherein the driver circuitcomprises a third transistor, wherein the third transistor comprises achannel formation region in a semiconductor substrate, wherein thesemiconductor substrate overlaps with the first transistor, the secondtransistor, the first light-emitting diode, and the secondlight-emitting diode with the fourth insulating layer therebetween, andwherein the first transistor and the first light-emitting diode overlapwith the third transistor.
 9. The display device according to claim 1,wherein the light emitted from the second light-emitting diode isextracted to the outside of the display device through the colorconversion layer and the coloring layer, and wherein the coloring layeris in contact with a third insulating layer which is in contact with thefirst light-emitting diode and the second light-emitting diode.
 10. Anelectronic device comprising: the display device according to claim 1;and at least one of an antenna, a battery, a housing, a camera, aspeaker, a microphone, and an operation button.
 11. A display devicecomprising: a first insulating layer; a second insulating layer; a firsttransistor; a second transistor; a first light-emitting diode; a secondlight-emitting diode; a third insulating layer over the firstlight-emitting diode and the second light-emitting diode; alight-blocking layer over and in contact with the third insulatinglayer; and a color conversion layer over and in contact with the thirdinsulating layer, wherein the first transistor is electrically connectedto the first light-emitting diode through a first conductor, wherein thesecond transistor is electrically connected to the second light-emittingdiode through a second conductor, wherein the first insulating layer isover the first transistor and the second transistor, wherein the firstlight-emitting diode and the second light-emitting diode are over thefirst insulating layer, wherein the color conversion layer is over andoverlaps the second light-emitting diode and the light-blocking layer,wherein the color conversion layer covers and is in contact with a sidesurface and a top surface of the light-blocking layer, wherein the colorconversion layer is configured to convert light emitted from the secondlight-emitting diode into a light having a longer wavelength, whereinthe first transistor and the second transistor each comprise a metaloxide layer and a gate electrode, wherein the metal oxide layer of thefirst transistor comprises a channel formation region of the firsttransistor, wherein the metal oxide layer of the second transistorcomprises a channel formation region of the second transistor, whereinthe gate electrode of the first transistor and the first light-emittingdiode overlap each other with the first insulating layer and the secondinsulating layer therebetween, wherein the gate electrode of the secondtransistor and the second light-emitting diode overlap each other withthe first insulating layer and the second insulating layer therebetween,and wherein the first conductor and the second conductor each comprise aconductive paste or a bump.
 12. The display device according to claim11, wherein the first transistor and the second transistor each comprisea back gate electrode.
 13. The display device according to claim 11,wherein the conductive paste comprises silver, carbon, or copper, andwherein the bump comprises gold or silver.
 14. The display deviceaccording to claim 11, further comprising a resin layer over thelight-blocking layer and the color conversion layer, wherein the resinlayer is in contact with a top surface of the light-blocking layer.